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TRANSCRIPT
Micromachined Interfaces for Medical and
Biochemical Applications
Patrick Griss
Microsystem Technology
Department of Signals, Sensors and Systems (S3)
Royal Institute of Technology (KTH)
TRTA-ILA-0201
ISSN 0281-2878
Submitted to the School of
Computer Science – Electrical Engineering – Engineering Physics (DEF),
Royal Institute of Technology (KTH), Stockholm, Sweden,
in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Stockholm 2002
Micromachined Interfaces for Medical and Biochemical Applications
ii
The front cover shows a micromachined array of sharp spikes manufactured by the author.
The spike array is used for the measurement of biopotentials in vivo, in particular for the
monitoring of the activity of the brain. The spikes reduce the high impedance characteristic
of the human outermost skin by painlessly penetrating into deeper layers.
Copyright 2002 by Patrick Griss
Printed by KTH Högskoletryckeriet, Stockholm 2002
Micromachined Interfaces for Medical and Biochemical Interfaces
iii
Abstract
The possibility to miniaturize medical devices by using micromachining techniques offers new
opportunities for maintaining health and curing of diseases. The main advantage of micromachined
devices is that they are minimally invasive when used during surgical, diagnostic or therapeutic
treatments. Trauma can be reduced which facilitates the healing process, increases comfort for the
patient, and reduces recovery time. Thus, the efficiency of medical treatments can be increased.
Microfluidic systems enable the miniaturization of analytical techniques in biochemistry. They
offer the possibility for chemical compounds occuring in organisms to be studied in parallel and in an
automated way at large scale. In addition, sensitivity and speed of an analysis can be considerably
increased. Microfluidic devices have thus become very important in the search for new therapies and
as diagnostic tools.
The purpose of this thesis is to explore micromachined devices that act as interfaces in medical and
biochemical applications. A central concept in all described interfaces is the use of tapered
projections in the micro scale (also referred to as microspikes, microneedles or microtips). The
fabrication of the structures is based on deep reactive ion etching technology that was adapted to each
individual application.
The first described interface is a skin applied electrode for electroencephalographic recording, i.e.
the measurement of the electrical activity of the brain. The electrode is a device, which electrically
interconnects the human body to measurement amplifiers.
The second described device deals with microfluidic transdermal interfacing, i.e. the transfer of
liquid through the outer skin layers. This includes the controlled injection of drugs into the skin.
The third discussed device is an electrospray ionization emitter tip, which allows the formation of
ions of unknown molecules, e.g. proteins or peptides. Once the molecules are ionized, they can be
introduced to a mass spectrometer for the detection of their mass. Thus, the emitter tip interfaces a
liquid sample solution to the mass spectrometer.
Micromachined Interfaces for Medical and Biochemical Applications
iv
"To fly as fast as thought, to anywhere that is," he said, "you must begin by knowing that you have
already arrived..."
(from Jonathan Livingston Seagull by Richard Bach)
Micromachined Interfaces for Medical and Biochemical Interfaces
v
Contents
1 List of Publications 1
2 General Introduction 52.1 Medical and Biochemical Microsystems 52.2 Outline of the Thesis 6
3 Technology Basis 73.1 Tapered Projections in the Micrometer Scale 73.2 Deep Reactive Ion Etching Bosch Process 93.3 Combination of Isotropic and Anisotropic Etching 1 0
4 Skin Anatomy and Physiology 1 34.1 Epidermis 1 34.2 Dermis 1 4
5 Micromachined EEG Electrode 1 55.1 Anesthesia in Surgical Procedures 1 55.2 The Origin of Biopotentials 1 65.3 Spiked EEG Electrode 1 7
6 Microfluidic Transdermal Liquid Delivery System 2 36.1 Microfluidic Transdermal Interface 2 46.2 Dosing & Actuation Unit 2 8
7 Micromachined Electrospray Emitter Tips for MassSpectrometry 3 37.1 Electrospray Ionization 3 37.2 Proteomics 3 47.3 Micromachined Electrospray Emitter Tips 3 5
8 Final Conclusions 4 1
9 Acknowledgements 4 3
1 0 Summary of the Appended Papers 4 5
1 1 Glossary 4 9
1 2 References 5 1
1 3 Paper Reprints 5 7
1
1 List of Publications
This thesis is based on following reviewed journal papers:
1 Micromachined Electrodes for Biopotential Measurements
P. Griss, P. Enoksson, H. K. Tolvanen-Laakso, P. Meriläinen, S. Ollmar, and G. Stemme,
Journal of Microelectromechanical Systems, vol. 10, pp. 10-16, 2001.
2 Micromachined Barbed Spikes for Mechanical Chip Attachment
P. Griss, P. Enoksson, and G. Stemme,
Sensors & Actuators: A Physical, vol. 95, pp. 94-99, 2002.
3 Expandable Microspheres for the Handling of Liquids
P. Griss, H. Andersson, and G. Stemme,
Lab-on-Chip, vol. 2, 2002.
4 Characterization of Micromachined Spiked Biopotential Electrodes
P. Griss, H. Tolvanen-Laakso, P. Meriläinen, and G. Stemme,
IEEE Transactions on Biomedical Engineering, Accepted for publication, 2002.
5 Side-Opened Out-of-Plane Microneedles for Microfluidic Transdermal Liquid Transfer
P. Griss and G. Stemme,
Submitted, 2002.
6 Development of Micromachined Hollow Tips for Protein Analysis Based on
Nanoelectrospray Ionization Mass Spectrometry
P. Griss, J. Melin, J. Sjödahl, J. Roeraade, and G. Stemme,
Submitted, 2002.
The contributions of the author to these papers are:
1 – 4 All fabrication, part of experiments, major part of writing.
5 All fabrication, all experiments, major part of writing.
6 Part of fabrication, part of experiments, major part of writing.
Micromachined Interfaces for Medical and Biochemical Applications
2
The work was also presented at following conferences:
1 Spiked Biopotential Electrodes
P. Griss, P. Enoksson, H. Tolvanen-Laakso, P. Meriläinen, S. Ollmar, and G. Stemme,
presented at The 13th Annual International Conference on Micro Electro Mechanical Systems,
Miyazaki, Japan, 2000.
2 Barbed Spike Arrays for Mechanical Chip Attachment
P. Griss, P. Enoksson, and G. Stemme,
presented at The 14th Annual International Conference on Micro Electro Mechanical Systems,
Interlaken, Switzerland, 2001.
3 Novel, Side Opened Out-Of-Plane Microneedles for Microfluidic Transdermal Interfacing
P. Griss and G. Stemme,
presented at The 15th Annual International Conference on Micro Electro Mechanical Systems,
Las Vegas, USA, 2002.
4 Liquid Handling Using Expandable Microspheres
P. Griss, H. Andersson, and G. Stemme,
presented at The 15th Annual International Conference on Micro Electro Mechanical Systems,
Las Vegas, USA, 2002.
5 Characterization of Micromachined Hollow Tips for 2D Nanoelectrospray
Massspectrometry
J. Sjödahl, P. Griss, J. Melin, Å. Emmer, J. Roeraade, and G. Stemme,
presented at the 15th International Symposium on Microscale Separations and Analysis,
HPCE 2002, Stockholm, Sweden.
Micromachined Interfaces for Medical and Biochemical Interfaces
3
In addition, the author has contributed to the following related journal papers:
1 Expandable Microspheres - Surface Immobilization Techniques
H. Andersson, P. Griss, and G. Stemme,
Sensors and Actuators B: Chemical, Accepted for publication, 2002.
2 Hydrophobic Valves of Plasma Deposited Octafluorocyclobutane in DRIE Channels
H. Andersson, W. van der Wijngaart, P. Griss, F. Niklaus, and G. Stemme,
Sensors and Actuators B: Chemical, vol. 75, pp. 136-141, 2001.
3 Low-Temperature Wafer-Level Transfer Bonding
F. Niklaus, P. Enoksson, P. Griss, E. Kälvesten, and G. Stemme,
Journal of Microelectromechanical Systems, vol. 10, pp. 525-531 2001.
4 A Variable Optical Attenuator Based on Silicon Micromechanics
C. Marxer, P. Griss, and N. F. de Rooij,
IEEE Photonics Technology Letters, vol. 11, p.233-235, 1999.
5
2 General Introduction
Microelectronics has allowed people to process information at unprecedented speed, at an
affordable cost and with computers that are portable. Micromachining methods that allow the
realization of the basic elements of microelectronics on a scale, or size, of micrometers are the keys
for the introduction of processors and memories in our daily lives.
The use of micromachining methods to fabricate microscaled sensors and actuators instead of
integrated circuits some 30 years ago has now developed into the area of microsystem technology.
Microsystems and –devices can process mechanical, fluidic, electrical, optical, thermal and radiative
signals, often integrated, on one single chip [1]. This area is about to complete the penetration of
microelectronics into our society and further enables people to interact with their environment and
their own physiology in a way that was not possible before.
2.1 Medical and Biochemical Microsystems
The possibility to miniaturize medical devices by using micromachining technology offers a whole
range of opportunities for the maintenance of health and the cure of diseases. The main advantage of
micromachined devices is that they are minimally invasive when used during surgical, diagnostic or
therapeutic treatments. Trauma can be reduced which facilitates the healing process, increases
comfort for the patient, and reduces recovery time. Thus, the efficiency of medical treatment can be
increased. Miniaturization also allows light and portable devices to be fabricated, an important issue
for diagnostic systems. This is in line with modern healthcare’s demand for comfortable, cheap, home
treatment and point-of-care diagnosis.
Microfluidic systems enable the miniaturization of analytical techniques in biochemistry and are
often referred to as micro total analysis systems or Lab-on-Chip. They offer the possibility for
parallelization and automation of the study of chemical compounds occurring in organisms at large
scale. In addition, sensitivity and speed of an analysis can be considerably increased. Microfluidic
devices have thus become very important as diagnostic tools and in the search for new therapies.
Micromachined Interfaces for Medical and Biochemical Applications
6
2.2 Outline of the Thesis
The purpose of this thesis is to explore micromachined interfaces for medical or biochemical
applications. A central concept is the use of tapered projections in the micro scale (also referred to as
microspikes, microneedles or microtips). The fabrication of these structures is based on deep reactive
ion etch technology that was adapted to each individual application. The fundamentals of the chosen
fabrication technology are described in chapter 3.
The first described interface is a skin applied electrode for electroencephalographic recording, i.e.
the measurement of the electrical activity of the brain. The electrode is a device, which electrically
interconnects the human body to measurement amplifiers. The work around this electrode is the
foundation of this thesis and was performed in collaboration with Datex-Ohmeda. Datex-Ohmeda is
the largest division of Instrumentarium Corporation, one of the world’s leading medical technology
companies. Datex-Ohmeda represents the group's core business, anesthesia and critical care. Datex-
Ohmeda is headquartered in Helsinki, Finland, and has manufacturing operations in Finland, Sweden
and the United States [2, 3].
The second described device deals with microfluidic transdermal interfacing, i.e. the transfer of
liquid through the outer skin layers. This includes the controlled injection of drugs into the skin.
The third discussed device is an electrospray ionization emitter tip, which allows the formation of
ions of unknown molecules, e.g. proteins or peptides. Once the molecules are ionized, they can be
introduced to a mass spectrometer for the detection of their mass. Thus, the emitter tip interfaces a
liquid sample solution to the mass spectrometer.
Both the EEG electrode and the microfluidic transdermal interface deal with the electrical,
chemical and mechanical properties of the skin. Skin anatomy and physiology are briefly described in
chapter 4.
7
3 Technology Basis
The devices explored in this thesis rely on tapered projections in the micrometer scale. Tapered
microprojections are often referred to as microneedles, microspikes or microtips. They can be hollow
or solid depending on their function.
Tapered projections have been used throughout history to solve various engineering problems, and
since the invention of the gramophone, their use is in the sub-millimeter scale. In this chapter an
overview on the different uses of micromachined projections in scientific and commercial applications
is given. Subsequently, the fundamentals of the fabrication technology employed to manufacture the
devices of this thesis are described.
3.1 Tapered Projections in the Micrometer Scale
Recently, activities in micromachining microprojections for various applications have increased.
Two fundamentally different fabrication principles have been used as illustrated in Figure 3.1:
(i) out-of-plane fabrication, where the resulting projection is protruding perpendicularly from
the plane of the substrate material (e.g. a silicon wafer), or
(ii) in-plane fabrication, where the resulting projection is “lying” in the wafer plane.
The choice between these two methods depends strongly on the final application of the device. If it
is desired to have a two-dimensional (2D) array of microprojections where both extensions in the x
and y directions contain several (even hundreds) of single projections, it is probably more efficient to
use the out-of-plane manufacturing method. This is because the array is formed at the same time as
the needles. If “long” projections are required, for example in the millimeter range, in-plane
fabrication is preferable. In this case 2D-array building requires an additional fabrication step, which
is not on wafer level and therefore time consuming. The advantage of in-plane needles is that there is
the possibility to manufacture electrical, microfluidic, or optical functionality on the needle shaft.
A literature study reveals that there are many areas where individual or arrays of tapered
microprojections are of interest. Table 3.1 is a summary on the use of microneedles, microtips and
microspikes for scientific and commercial applications. The microprojections are classified
corresponding to the physical nature of their interaction with the environment, the type of interaction,
and the purpose of their use.
Micromachined Interfaces for Medical and Biochemical Applications
8
OP
TIC
AL
Sub
wav
elen
gth
emis
sion
Supe
r re
solu
tion
mic
rosc
opy
SNO
M3 [
4]
com
bine
d
SNO
M3 a
nd
AFM
1 [5]
-
Ele
ctri
c fi
eld
conc
entr
atio
n
Par
ticl
e ge
nera
tion
elec
tros
pray
em
itter
nozz
les
[6, P
aper
6]
prin
t-he
ad [
7]
fiel
d em
itter
s [8
, 9]
X-R
ay s
ourc
e [1
0]
elec
tros
pray
em
itter
need
les
[11,
12]
EL
EC
TR
ICA
L
Tun
nelin
g
curr
ent
Surf
ace
prof
iling
STM
2 [13
]
-
Surf
ace
cont
acti
ng
Surf
ace
prof
iling
AFM
1 [14
, 15,
16]
com
bine
d A
FM1
and
SNO
M3 [
5]
-
Tra
nsfe
r of
mat
ter
tran
sder
mal
liqui
d tr
ansf
er[1
7, 1
8, 1
9,Pa
per
5]
DN
A tr
ansf
erin
to ti
ssue
[20
,21
]
tran
sder
mal
liqui
d tr
ansf
er[2
3,24
,25,
26, 2
7]
mic
rodi
alys
is[2
8]
Tra
nsfe
r of
ener
gy
stim
ulat
ion/
re-
cord
ing
from
visu
al c
orte
x [2
9]
EE
G r
ecor
ding
[Pap
er 1
, Pap
er 4
]
deliv
ery
of c
hem
ical
s an
d re
cord
ing
ofel
ectr
ical
res
pons
e [2
2]
elec
tric
alre
cord
ing
in th
eC
NS4 [
30]
ther
mal
mar
king
and
mon
itori
ng o
fne
ural
tiss
ue [
31]
coch
lear
impl
ants
[32]
ME
CH
AN
ICA
L
Surf
ace
pene
trat
ion
Surf
ace
mod
ific
atio
n
“Mill
iped
e”da
tast
orag
e[33
]
-
Out
-of-
plan
e
fabr
icat
ion
In-p
lane
fabr
icat
ion
1 Ato
mic
For
ce M
icro
scop
e 2 Sc
anni
ng T
unne
ling
Mic
rosc
ope
3 Sc
anni
ng N
ear-
Fiel
d O
ptic
al M
icro
scop
y
4 Cen
tral
Ner
vous
Sys
tem
TA
BL
E 3
.1:
OV
ER
VIE
W O
F T
HE
USE
OF
MIC
RO
MA
CH
INE
D T
APE
RE
D P
RO
JEC
TIO
NS
IN S
CIE
NT
IFIC
OR
CO
MM
ER
CIA
L A
PPL
ICA
TIO
NS.
TH
E C
ITE
D E
XA
MPL
ES
AR
E C
LA
SSIF
IED
AC
CO
RD
ING
TO
TH
E P
HY
SIC
AL
NA
TU
RE
OF
TH
E I
NT
ER
AC
TIO
N W
ITH
TH
E E
NV
IRO
NM
EN
T, T
HE
TY
PE O
F IN
TE
RA
CT
ION
AN
D T
HE
PU
RPO
SE O
F T
HE
IR U
SE.
FO
R E
AC
H E
XA
MPL
E, A
T L
EA
ST
ON
E R
EFE
RE
NC
E I
S A
NN
OT
AT
ED
.
Phy
sica
l nat
ure
of th
e
inte
ract
ion
Typ
e of
Int
erac
tion
Pur
pose
of
use
App
licat
ion
exam
ples
Technology Basis
9
3.2 Deep Reactive Ion Etching Bosch Process
Design and function of microdevices is strongly related to their fabrication process. There is a vast
variety of microfabrication technologies. The most commonly used are derived from the fabrication
technologies developed to fabricate integrated circuits [1, 34]; the most powerful of these is
photolithography. However, there are complementary and alternative methods such as polymer
replication [35], soft lithography [36], polymer lamination [37], and high-aspect ratio photosensitive
resins [38]. Therefore, microdevices need not be made from only silicon. Yet, silicon has some
advantages compared to glass or polymers, for example. Aside from the possibility to integrate
circuitry directly onto the transducer’s substrate, the excellent mechanical and electrical properties are
often very important for the performance of the microdevice.
Masking Material
Substrate Material(e.g. Silicon)
Out-of-Plane In-Plane
z
x y
Figure 3.1: Schematic representation of the fabrication principles of micromachined tapered projections.
The development of anisotropic etch tools for silicon that allow etching of deep holes or high free
standing structures are of considerable interest for microsystem technology and in particular for this
thesis [39, 40, 41]. The anisotropic etching of high-aspect ratio structures in silicon is often referred to
as deep reactive ion etching (DRIE).
Commercially available anisotropic etchers include inductively coupled plasma (ICP) etchers.
They rely on a high-density plasma source and an alternating process of etching and protective
polymer deposition to achieve aspect ratios up to 30:1 [39]. This approach utilizes an etching cycle
using only SF6 gas and then switches to a passivation cycle using only C4F8, as schematically
represented in Figure 3.2. During the etching cycle, the passivating film is preferably removed from
the bottom of the trenches due to directional ion bombardment. The final result is an anisotropic etch
profile that exhibits scalloping. As it becomes clear from the drawings, the actual anisotropic etch
consists of a large number of isotropic etches. Controlling plasma parameters as well as gas
pressure/flow parameters, the etch characteristics and profile can be controlled [42].
Micromachined Interfaces for Medical and Biochemical Applications
10
C F84
SF6
SF6
SF6
Silicon
Silicon dioxide
C F84
Etch
Passivation
Passivation
Etch
Et Cetera ...
Figure 3.2: Everything You Always Wanted to Know About Bosch’s Process (But Never Dared to Ask).
By suppressing switching of etch and passivation gases, the equipment can be run with continuous
flow of SF6 or C4F8. Thus, using only SF6 it is possible to etch isotropically and using only C4F8 it is
possible to deposit Teflon like layers on three-dimensional structures. All structures presented in this
thesis were micromachined using a DRIE capable ICP from Surface Technology Systems.
3.3 Combination of Isotropic and Anisotropic Etching
The combination of isotropic and anisotropic etch steps is a fundamental part of the fabrication of
the devices presented in this thesis. It was observed that if a silicon dioxide mask is underetched and
subsequently anisotropically etched, the resulting cross-section of the structure corresponds to the
mask (Figure 3.3 a)-b)). It was further observed that vertical walls of DRIE etched high aspect ratio
silicon structures stay vertical during an isotropic plasma etch (Figure 3.3 b)-c)). Based on these
observations, a variety of tapered out-of-plane projections were manufactured for different
applications.
Technology Basis
11
Depending on the application, it is possible to shape the geometry of the projections. Overall
dimensions, sharpness of the tip, “smoothness” of the shape, and mechanical stability can be
controlled by controlling the length of the various etch steps, as summarized in Table 3.2.
Isotropic etch 1 Anisotropic etch 1 Isotropic etch 2
D
sβ
H
h
d
Anisotropic etch 2
Silicon
Silicon dioxide
a) b) c) d)
Figure 3.3: Combination of isotropic and anisotropic etch steps to achieve out-of-plane, tapered, silicon
projections. The shown dimensions in d) are controlled via process parameters. Table 3.2 is a summary of how
processing time influences the represented geometrical dimensions.
TABLE 3.2:
PROVIDED THAT THE ICP SETTINGS ARE SUCH THAT ANISOTROPIC ETCH RESULTS IN VERTICAL PROFILES, THE
LENGTH OF THE DIFFERENT ETCH STEPS INFLUENCE THE SHAPE AND DIMENSIONS OF THE RESULTING MICROSPIKE.
ISOTROPIC 1 AND ISOTROPIC 2 ARE THE ETCH TIMES OF THE FIRST AND SECOND ISOTROPIC ETCH, RESPECTIVELY.
ANISOTROPIC 1 AND ANISOTROPIC 2 ARE THE ETCH TIMES OF THE FIRST AND SECOND ANISOTROPIC,
RESPECTIVELY.
Etch timeGeometrical dimension
(refer to Figure 3.3) Isotropic 1 Anisotropic 1 Isotropic 2 Anisotropic 2
s ↑ ↓↓↓↓ N/I ↓↓↓↓ N/I
D ↑ N/I N/I ↓↓↓↓ N/I
d N/I N/I N/I N/I
H ↑ ↑↑↑↑ ↑↑↑↑ ↑↑↑↑ N/I
h ↑ ↑↑↑↑ ↑↑↑↑ ↑↑↑↑ ↑↑↑↑
β ↑ ↓↓↓↓ N/I ↑↑↑↑ N/I
↑... increasing value N/I...no influence ↓… decreasing value
13
4 Skin Anatomy and Physiology
In this chapter basic anatomy and physiology of the human skin is explained. Understanding of the
skin is of importance for two of the described devices, i.e. the spiked electrode and the microfluidic
transdermal interface.
The seemingly inert appearance of the skin hides its multifunctional properties. It contains the
body’s fluids and tissues and is a protective layer against physical, chemical and biological attacks. It
is a receptor of tactile, painful and thermal stimuli. It regulates heat loss from the body and plays an
important role in the control of immunity via dendritic cells [43, 44]. The skin has a layered structure
and can be subdivided into the Dermis and the Epidermis. Underneath the Dermis is the subcutaneous
tissue, which mainly consists of fat. Within the Dermis are varying numbers of hair follicles and
sweat glands.
4.1 Epidermis
The Epidermis can be divided into three important layers: the outer most Stratum Corneum (also
called the horny layer) composed of fibrous keratinized dead cells; a layer of Living Epidermis; and
the basal layer or Stratum Germinativum, as schematically represented in Figure 4.1.
The basal layer gives rise to living epidermal cells by division of the cells within the Stratum
Germinativum. The epidermal cells possess a high metabolic rate and gradually move upward to
transform into keratinized cells, which are flat, compact and dehydrated. The Epidermis is constantly
renewing itself approximately once every 30 days.
Located within the Stratum Germinativum and the Living Epidermis are dendritic cells, e.g.
melanocytes and Langerhans cells. They are the initiators and modulators of the immune response
[44].
The Stratum Corneum imparts the major barrier function to skin, which prevents loss of water and
invasion by microorganisms as well as a large number of chemical agents. At the same time the
Stratum Corneum provides a flexible, tough mechanical protection. Because of its dehydration it is
electrically isolating. There are no blood vessels in the Epidermis; nutrition diffuses outwards from
the rich blood supply of the Dermis, and there are no sensory organs [43, 45].
Micromachined Interfaces for Medical and Biochemical Applications
14
The Stratum Corneum is about 10-15 µm thick on the forearm for example, while it is twice as
thick on the back and hundreds of microns thick under the feet. The Living Epidermis is around
150 µm thick.
4.2 Dermis
Figure 4.1 shows that the Dermis is much thicker than the Epidermis and mainly consists of
collagen, which builds a supportive connective tissue matrix. It supplies the Epidermis with vital
nutrients and oxygen via a network of blood capillaries. It is the connective tissue of the Dermis that,
when properly prepared from animals by tanning, becomes the tough leather that has many uses [45].
The Dermis contains many sensory organs for touch, pressure, pain and temperature and is richly
supplied with specialized nerve endings, nerve fibers and pressure sensors.
The dermal thickness varies between 600 µm on the eyelids to several millimeters on the back,
palms, and soles.
Stratum Corneum
Langerhans Cell
Stratum Germinativum
Blood Capillaries
Blood Capillaries
Hair
Dermis
SubcutaneousTissue
Epidermis
Sweat Gland
Sensory Organs andNerve Endings
Melanocyte
Living Epidermis
Figure 4.1: Layered architecture of the skin along with organs embedded in the skin. Sharp projections that
extend into the Stratum Germinativum and not further can be expected to be painfree since there are no sensory
organs or nerve endings that could trigger any sensation of pain. Also, such projections cannot cause bleeding
since blood capillaries cannot be reached.
15
5 Micromachined EEG Electrode
In neurological and cardiological applications, the classical methods of recording biopotentials
using skin applied electrodes are well established. Biopotentials are produced by the electrochemical
activity of excitable cells, e.g. nerve cells in the brain. An excited nerve cell creates an electrical
current of ions (e.g. Cl-) which induces a measurable potential on the surface of the body, i.e. the skin
[46].
Examples of biopotential recordings include monitoring of heart activity (ECG) and diagnosis of
epilepsy via electroencephalography (EEG). Among these methods EEG is the most difficult to
record since signal levels are very low [47].
Towards the end of the 20th century, sophisticated processing of EEG data emerged. Researchers
started to better understand how to efficiently extract information from the very complex and fractal
EEG signals [48]. This information can be used in patient monitoring via EEG. One example is
determining the level of sedation of patients during surgery.
In this chapter a background on anesthesia monitoring is given. Subsequently, the design,
fabrication and testing of micromachined electrodes measuring EEG and acting as interfaces between
the patient and the monitoring equipment is shown.
5.1 Anesthesia in Surgical Procedures
Most surgical procedures cause pain and traumatic experiences and are stressful for the patient. To
spare the patient from pain and stress, anesthetists and monitoring equipment are needed to protect the
patient. This entails removing the sensation of pain by administering anesthetic agents and pain
medication in optimal combinations, often also producing unconsciousness. Anesthetists also give
medication to achieve a suitable degree of muscle relaxation to ensure optimal surgical conditions.
Often, the above-mentioned delivery of anesthetic agents disables spontaneous breathing. A ventilator
therefore maintains the transport of breathing gases to and from ambient air.
Until now, anesthetists have had no direct means of assessing a patient’s level of consciousness.
They have generally relied on recommended dosages and on indirect indicators of consciousness as
blood pressure or heart rate [2].
Micromachined Interfaces for Medical and Biochemical Applications
16
EEG Electrodes
Figure 5.1: Scene representing the Datex-Ohmeda S/5™ Anesthesia System, which integrates management of
the entire anesthesia process from pre-operative assessment to post-operative analysis. EEG electrodes are the
sensor elements allowing brain activity to be monitored for direct measurement of the level of unconsciousness.
The complete S/5™ system can include ventilation and monitoring of parameters like airway gases, cardiac
function (ECG), circulation, lung mechanics, etc. (Source: [2], with permission)
Recent studies by Datex-Ohmeda, Helsinki, Finland, have suggested that parameters corresponding
to the level of complexity of the EEG signal indicate the anesthetic effect, e.g. the level of hypnosis
[48]. This means that measuring EEG of a patient during surgery offers the possibility to directly
assess the effect of anesthetics, i.e. monitor the patient via EEG recordings as shown in Figure 5.1.
Roughly 75 million patients receive general anesthesia every year in main industrialized countries.
Monitoring these people with EEG results in several advantages for patients, hospitals and society:
(i) the risk assessment of surgical awareness (i.e. the unintentional return of consciousness
during surgery) can be improved
(ii) the patient’s quality of recovery is improved
(iii) the patient’s time in the operating room and the post anesthesia care unit is reduced
(iv) the patient wakes up faster
(v) drug use and costs can be reduced.
5.2 The Origin of Biopotentials
Biopotentials are produced by the electrochemical activity of excitable cells that are found in
nervous, muscular and glandular tissue. Active transport of Na+ out of the cell and of K+ into the cell
as well as different permeabilities of Na+, K+ and Cl- result in a negative net charge of the cell interior
and a negative membrane potential, as explained in Figure 5.2 [49]. When excited, for example by
depolarizing the cell membrane with an electrical impulse, this equilibrium is momentarily disturbed.
Micromachined EEG Electrode
17
Due to the voltage dependence of the membrane permeabilities, Na+ rushes into the cell, leaving the
cell further depolarized. This potential is referred to as action potential and is stopped at about
+20 mV since the increase of the Na+ permeability is also time dependent and relatively short [46].
Action potentials propagate along an excitable cell and therefore the cell acts like a signal carrier.
A solenoidal ionic current is induced in the interstitial liquid surrounding the cell, as shown in Figure
5.2. This current causes electrical potential differences in the human body fluids and is measurable on
the skin [47].
The brain is mainly made up of nervous tissue. Therefore, its activity is reflected on the skull as a
changing pattern of potential differences that can be recorded from specific locations on the skull, in
particular on the forehead. Such a recording is referred to as electroencephalograph (EEG).
ActiveRegion
Ionic Current
Direction of signal propagation
- - - - - -- - ++ + ++ + ++ + ++ - - - - -- - - - --
- - - - - -- - ++ + ++ + ++ + ++ - - - - -- - - - --
Net charge ofthe cell interior
Mem
bran
e po
tent
ial [
mV
]
ExciteableCell
Cell membrane
-80
0
+20
Figure 5.2: Signal propagation along the excitable cell.
The measurement of biopotentials on the skin or within tissue in vivo involves an electrode-
electrolyte interface [46, 50]. The measurement is an electrochemical process, which transforms an
ionic current into an electronic current. Only electronic currents can be used in analyzing electronics.
For traditional skin applied electrodes, an electrolytic gel is applied between the metal electrode and
the skin acting. Because of the gel they are referred to as “wet” electrodes.
5.3 Spiked EEG Electrode
Inappropriate electrode technology was one impediment to using EEG in routine patient
monitoring. As the signals to be measured are very low, conventional EEG electrodes require
preparation of the skin (cleansing with alcohol, abrasion of the outer skin layer) and/or the application
of electrolytic gel at the electrode application site. Both skin preparation and electrolytic gel are
intended to reduce the excellent electrical isolating characteristics of the Stratum Corneum (refer to
chapter 4). Often, conventional electrodes are not suited for clinical use because the application of the
Micromachined Interfaces for Medical and Biochemical Applications
18
electrode is time consuming and the signal quality depends on the skill of the nurse. Research with the
goal to eliminate skin preparation has resulted in dry (i.e. without use of electrolytic gel) ceramic
based NASICON electrodes [51], on chip amplified dry electrodes [52, 53], and the wet Zipprep™
electrode [54].
no electrolytic gel
electrolytic gel
Stratum Corneum
Epidermis
Dermis
spikes
wire to measurement electronics
5 - 10 m50 - 100 m
Figure 5.3: Artistic drawing comparing the concept of the spiked electrode (left) to the concept of conventional
electrodes (right).
In collaboration with Datex-Ohmeda Division, Instrumentarium Corp., Helsinki, Finland, a novel
type of EEG electrode was developed. An array of microscaled, sharp spikes penetrates the Epidermis
to circumvent the isolating characteristics of the Stratum Corneum. This novel principle of a skin
applicable biopotential electrode is schematically represented in Figure 5.3 [Paper 1].
5.3.1 Objectives
Low electrode-skin impedance is the main goal. The lower the impedance, the more effectively the
electrical activity of the brain is measured since the charges from the tissue are effectively transferred
into the analyzing electronics. Another aspect of low electrode-skin impedance is the minimization of
noise in EEG recordings since impedance mismatch of different electrodes is reduced [55].
Another demand on the spiked EEG electrode is to avoid any skin preparation and electrolytic gel
to achieve very easy use and fast, uncomplicated application on the skin.
The minimization of noise resulting from electrochemical processes in/on the skin is critical too,
both in time, i.e. fast stabilization after application on the skin, as well as in amplitude.
High patient acceptance can be reached by maximizing the comfort for the patient. The spiked
EEG electrode should therefore be more comfortable than other products, especially wet electrodes.
Last but not least, the electrode must not create a hazard to the patient. This includes physical as well
as chemical factors, e.g. the electrode should be highly resistant to mechanical stress and it should not
be toxic.
Refer to Table 5.1 for a summary of the objectives of the micromachined EEG electrode.
5.3.2 Prototype Devices
DRIE fabrication of silicon was chosen to fabricate spike arrays [Paper 1]. Figure 5.4 shows a
scanning electron microscope (SEM) of a chip containing an array of spikes 160 µm in length. These
types of spikes were pressed into human skin in vitro and Figure 5.5 shows the photograph of a section
Micromachined EEG Electrode
19
of the penetration site of a spike. The figure shows that the Stratum Corneum is totally disrupted and
that the spike penetrates into the living Epidermis, slightly further extending into the Dermis.
Experience showed that spikes of up to 200 µm length were painfree and comfortable for the patient
when applied on the forehead. In some cases a somewhat tickling feeling was reported during the
application of the electrode on the forehead, which probably results from the edges of the chip rather
than the spikes itself. Touching the micromachined surface of a spiked electrode feels like touching
fine sandpaper.
TABLE 5.1:
LIST OF THE MAIN OBJECTIVES OF THE MICROMACHINED EEG ELECTRODE IN DECREASING ORDER OF PRIORITY.
1. Minimize skin-electrode impedance caused by the Stratum Corneum
2. Avoid skin preparation
3. Avoid electrolytic gel
4. Minimize electrical noise levels
5. Maximize comfort for the patient (pain free, minimize irritation, minimize tissue trauma)
6. Maximize the mechanical stability of the microneedle (array)
7. Disposability
Figure 5.4: SEM image of silver coated spikes. The
length of the spikes is 160 µm, the diameter is 40 µm.
Note the through hole in the center of the image
which is electrically interconnecting the front to the
back side of the chip.
Figure 5.5: Section of human skin previously
penetrated by microspikes, 160 µm in length in
vitro. The penetration depth is approximately
100 µm and the spike apex extends slightly into the
dermis. The dotted line represents the limit between
Dermis and Epidermis.
Micromachined Interfaces for Medical and Biochemical Applications
20
The developed fabrication process allows the spikes to be modeled. The spikes show a sharp tip
apex that favor easy penetration into the skin, a sturdy shaft and a rounded base, which decreases
stress concentration when the spike is loaded with shear forces.
The spikes are coated with a silver-silverchloride (Ag-AgCl) double layer to reduce polarizablity of
the electrode-electrolyte interface, where the electrode is the spike surface and the electrolyte the
tissue in the Living Epidermis [46, 50, 56, 57, Paper 1, Paper 4].
A suitable package consists of a thin, circular disk (e.g. a double sided printed circuit board) glued
to the back side of the spike array chip, as shown in Figure 5.6. A ring-shaped adhesive tape is fitted
onto the disk and firmly adheres the disk and the electrode chip to the skin. The spiked electrode is
connected to the analyzing electronics by a lead wire. As the wire diameter is larger than the length of
the spikes, it must be attached to the back side of the package to avoid any influence on the proper
penetration of the spikes into the skin. Dry use of the spiked electrode without any skin preparation
presupposes that the electrode chip is securely attached to the skin allowing the spikes to pierce the
Stratum Corneum and penetrate the Epidermis. Choosing an appropriate tape with very good adhesion
performance is therefore of importance.
Lead wire
PCB
Double sidedcopper pad
Double sidedmedical tape
Spike array chip
Conductingepoxy
Solder
a) b)
Figure 5.6: Packaging of spiked EEG electrodes. a) A drawing of the exploded view of the package. The
double-sided printed circuit board (PCB) features electrically interconnected copper pads on both sides. The chip
is glued to the lower pad using conductive epoxy; the lead wire is soldered to the upper pad. b) Two photographs
(from the front – and the back side) of the electrode assembly ready to be applied on the skin. The scale is 1 cm.
Pre-clinical tests were done at Datex-Ohmeda and at the Microsystem Technology group involving
volunteer test persons [Paper 4]. During these tests we observed that no skin preparation is needed
prior to the application of the spiked electrode on the skin and no electrolytic gel is required during the
Micromachined EEG Electrode
21
use of the electrode. The electrode-skin impedance (ESI) is low and stable. Both ESI magnitude and
stability are in the range of commercially available high performance electrodes using electrolytic gel,
e.g. Zipprep™ or 3M Red Dot (see Table 5.2). The microscaled spikes do not create a sensation of
pain and especially during long-term measurements, the spiked electrode is more comfortable for the
patient than wet commercial electrodes. Due to the mechanical strength of the spikes, handling of the
spiked electrodes is convenient and does not require special care by the personnel.
Figure 5.7 shows a 5 second epoch of a typical EEG recorded with spiked electrodes. The shown
signal is representative when monitoring anesthesia via EEG. In the case of the shown EEG, the
subject is still conscious and thus unwanted signals, for example eye blinking or muscle activity, are
recorded too. Preliminary analysis of the calculated entropy of the recorded EEG does not show any
significant difference between the spiked and the wet control electrode. Further, visual examination of
both signals shows that they are similar enough to be used for entropy determination [48].
The spiked electrode project has won the national final of the Swedish Innovation Cup 2000,
organized by Skandia, a global savings company, and Dagens Industri, an industrial news paper. The
prize awards innovative research projects with commercial interest.
Time [s]
Spiked Electrode
Control Electrode
15
10
5
0
-5
-100 1 2 3 4 5
Figure 5.7: Comparison of the EEG recorded by spiked electrodes and a commercial control electrode
(ZipprepTM). The total epoch is 5 s long; the sampling rate is 100 Hz. The three distinct transients correspond to
eye blinks. As long as the patient is still awake, the shown EEG is representative during the monitoring of
anesthesia levels.
In an attempt to simplify the electrode packaging, a fabrication process was developed that allows
barbs to be created at the tip of the spikes, as shown in Figure 5.8 [Paper 2]. The barbs allow a spike
to penetrate the skin but offer resistance to detaching forces. Thus there is the possibility to attach a
spike array to the skin without using adhesive tape. Although the barbs significantly increase the
Micromachined Interfaces for Medical and Biochemical Applications
22
required detachment forces compared to spike arrays without barbs, the resulting attachment force
created by the barbs was not large enough to be used in clinical applications
TABLE 5.2:
COMPARISON OF THE MAXIMAL AND MINIMAL MEASURED TOTAL IMPEDANCE OF COMMERCIAL 3M RED DOTTM
AND SPIKED ELECTRODES AT 1000 HZ AND APPROXIMATELY 0.5 HZ. THE SPIKED ELECTRODES CONSIDERED IN
THIS TABLE ARE CHLORIDED AND HAVE A SIZE OF 3X3 MM OR LARGER.
Max kΩ Min kΩ
R at 1000Hz Dry Spiked 4.23 0.65
R at 1000Hz Wet 3M Red Dot 3.17 0.5
R at 0.6 Hz Dry Spiked 87 16
R at 0.5 Hz Wet 3M Red 177 14.8
.
Figure 5.8: (left) SEM image of an array of barbed spikes. (right) Photograph to demonstrate the attachment of a
chip to human skin by barbed spikes (the same type of barbs as shown in the left image). The attachment is such
that the skin is pulled upwards by the barbed array without the help of any other adhesion enhancer.
In this work we substantiate that spiked electrodes have the potential to be used as sensors
recording EEG for anesthesia monitoring. EEG monitoring methods have increasing impact on
patient treatment in hospitals and represent a considerable market potential.
Future work will include the transfer of the microneedle technology to industrial manufacturing
methods for high volume production. Based on initial tests, it appears that plastic injection is a very
plausible candidate as such a production method. However, silicon processing will always have an
important position in the fabrication process. It is from silicon master structures that the moulds are
electroplated, which is a well-proven technology known from the compact disk fabrication lines.
23
6 Microfluidic Transdermal Liquid Delivery
System
Biotechnology and modern pharmaceutical technology enable a new generation of macromolecular
compounds with great therapeutic promise, including recombinant protein drugs [58]. Oral delivery is
not a realistic option for these molecules because of their degradation in the stomach, intestine or liver.
Alternative delivery routes must be found.
If drug volume and subcutaneous tolerability are acceptable, compounds that cannot be delivered
orally will most often be administered by injection. The use of standard hypodermic needles in
conjunction with syringes raises concerns related to comfort for the patient and thus to their suitability
for home treatment and self-injection. Therefore, it seems that optimization of the delivery of drugs
via the transdermal route is very important for modern therapies.
Large molecules, such as proteins or DNA chains, can not be delivered through the skin without
penetration enhancement since the excellent barrier characteristics prevented transport in
therapeutically relevant rates [59]. In contrast to low molecular weight entities used in patches for
smoke cessation, for example.
In future, many modern therapies will be vaccinations. Vaccines currently under development
include peptide drugs that raise high-density lipoproteins (i.e. “good cholesterol”) as well as
immunotherapies against Alzheimer’s disease and cancer [60, 61]. Another trend is visible regarding
the delivery route. In 2001, out of 129 drug delivery candidate products under clinical evaluation in
the USA, 51 were transdermal or dermal systems [62].
In this chapter micromachined interfaces are explored, which interconnect the lower skin layers or
hypodermic tissue with the outer world via microchannels and thus efficiently enhances the transport
of large molecules into or from the tissue. The interface also contains a unit, which accurately doses
and actuates the liquid to be delivered.
The transdermal liquid delivery system is subdivided in two sub-systems:
(i) the microfluidic transdermal interface (MTI) which creates physical pathways for the
liquid through the Stratum Corneum using microneedles and
(ii) the dosing and actuation unit (DAU) which is pumping the appropriate amount of liquid
through the MTI at the appropriate moment.
Micromachined Interfaces for Medical and Biochemical Applications
24
Both sub-systems are sought to be disposable devices and it is very important that they can be
sterilized by standard methods.
6.1 Microfluidic Transdermal Interface
6.1.1 Objectives
An important objective of the MTI is reduction of pain for the patient compared to syringe
injections. Reducing pain in transdermal drug delivery increases patient acceptance and thus increases
market potential for a liquid transdermal therapy. In addition, the MTI should be minimally invasive
to reduce trauma caused in the skin during liquid injection. Reducing trauma reduces the risk for
infection and speeds up healing processes.
The concept of the MTI is based on the creation of physical pathways, i.e. channels, into the tissue
below the Stratum Corneum. Low flow resistance of these channels enables effective liquid transfer
through the Stratum Corneum. During the injection, the tissue must absorb the liquid injected through
the channels, which results in a flow resistance created by the tissue. Therefore, the larger the delivery
area of the microneedles, the lower the flow resistance created by the tissue.
Penetration of the microneedles into the skin is a prerequisite for the described transdermal liquid
delivery. Therefore it is important to aim for sharp microneedles that offer smooth penetration into the
skin.
Lastly, a high resistance to mechanical stress is critical. The microneedles should not break while
inserted into the skin to avoid harm to the patient.
Table 6.1 is a summary of the objectives of the MTI.
TABLE 6.1:
LIST OF THE MAIN OBJECTIVES OF THE MICROFLUIDIC TRANSDERMAL INTERFACE IN DECREASING ORDER OF
PRIORITY.
1.Minimize the resistance of the Stratum Corneum to the transdermal delivery of large
molecules
2. Significantly reduce pain compared to standard injection
3. Minimize tissue trauma caused by the delivery
4. Minimize the mechanical resistance of the skin to microneedle penetration
5. Maximize the mechanical stability of the microneedle (array)
6. Disposability
Microfluidic Transdermal Liquid Delivery System
25
6.1.2 Prototype Devices
DRIE technology presented in chapter 2 was used to manufacture prototypes of MTI chips as
shown in Figure 6.1 [Paper 5]. The microneedles are approximately 200 µm long and feature side
openings and a solid tip apex.
Figure 6.1: Side opened microneedles approximately 200 µm in length.
Let us consider hollow tip opened microneedles presented elsewhere and schematically shown in
Figure 6.2 a) [18]. As it is desirable to minimize the flow resistance through the device, the bore in
the needle should be as large as possible. This creates a problem because the larger the bore, the more
probable it is to actually clog the bore during penetration of the skin. In other words: instead of
inserting the needle smoothly into the skin, the bore literally stamps a piece out of the skin. To avoid
this problem, the diameter of the bore can be reduced, which then again increases flow resistance in
the device and reduces delivery area. Another disadvantage of this design is that the tip opening is
never larger than the cross-section of the bore, which means that the drug delivery area in the tissue is
limited by the bore cross-section.
Efforts were made to overcome the “cookie cutter effect” of tip opened microneedles by taking
standard hypodermic needles as examples, as shown in Figure 6.2 b) [17, 19]. This increases delivery
area and creates a sharp edge, which facilitates skin penetration. The needle is slicing the skin with
the protruding edge before the bore enters the Stratum Corneum.
The side-opened microneedle design shown in this thesis features a novel approach, as illustrated in
Figure 6.2 c). The delivery area is at the shaft of the microneedle instead of at the tip apex, thus
avoiding the “cookie cutter effect”, even when large bore diameters are used. A slender shape of the
microneedle in combination with the sharp tip apex and four blades cutting into the tissue offer low
resistance during skin penetration. The piercing characteristics of this needle design into aluminum
foil is very good, the aluminum is sliced in a well-defined manner, as depicted in Figure 6.3. The
author believes that the side-opened microneedles penetrate into the skin in a similar, well-defined
manner. Because of the out-of-plane configuration of the design, two-dimensional arrays of needles
Micromachined Interfaces for Medical and Biochemical Applications
26
are fabricated on wafer level. This is an advantage for mechanical stability: A needle array inserted
into the skin is much more resistant to mechanical stress than one single in-plane needle. The
microneedle array as shown in Figure 6.1 was inserted into the skin and removed from the skin
repeatedly without observing any damage.
Figure 6.5 illustrates that the developed process technology makes it possible to design a delivery
area, which is adapted to a specific application. The size of the delivery area is independent of the
bore diameter and it can be positioned along the needle shaft. The delivery area of the needles shown
in Figure 6.1 is twice as large as it would be for a tip opened needle having the same bore diameter.
Path of the injected liquid
a) b) c)
Tip opening Oblique opening Side opening
Figure 6.2: Schematic drawing of the vertical cross-section of different hollow microneedle concepts. (a)
Suffers from high clogging probability during penetration into the skin. (b) Shows a schematic drawing of a
microneedle with an oblique opening, much like standard hypodermic neeldes. (c) Is the side opened
microneedle concept: low clogging probability combined with low flow resistance and good penetration
capabilities.
Figure 6.3: SEM of a MTI piercing 10 µm thick aluminum foil. Note the well-defined manner the needle cuts
the aluminum.
Microfluidic Transdermal Liquid Delivery System
27
200
300
400
500
600
700
800
900
18
20
22
24
26
28
30
32
2 3 4 5 6 7
Hypodermic Needle Diameter [µm]
Hypodermic Needle Gauge
Hyp
oder
mic
Nee
dle
Dia
met
er [
µm
]
Hypoderm
ic Needle G
auge
Chip Side Length [mm]
Figure 6.4: Comparison of the MTI chip to standard hypodermic needles. (left) SEM photograph of a 3×3 mm MTI
chip and two hypodermic needles of different diameters (0.36 mm and 0.9 mm). The semi-spherical “bump” at the
right corner of the MTI chip is conductive epoxy required for fixation of the chip to the sample holder. (right)
Relation between the side length of a MTI chip and the diameter of a hypodermic needle, both having the identical
total area of the bore cross-section.
sr t
Figure 6.5: The left schematic drawing illustrates some dimensions (e.g. r, s, and t) that the designer of the side-
opened microneedle is free to choose, in addition to the total length. The right SEM image shows that the size of
the delivery area can be pushed to the base of the needle.
It is interesting to compare the effective cross-sectional area of the liquid channel into the skin
created by standard hypodermic needles and the MTI, as shown in Figure 6.4. The diameter of a
hypodermic needle can be represented both with metric diameter and the needle gauge. A 3 × 3 mm
MTI chip as shown creates a channel into the skin which has approximately the same cross-sectional
area as a gauge1 27 needle for insulin delivery.
Vaccinations are well suited for micromachined needle arrays. In section 4.1 we have seen that
dendritic cells, which are initiators and mediators of the immune response, can be found in the Living
1 gauge 27 corresponds to a needle diameter of 360 µm
Micromachined Interfaces for Medical and Biochemical Applications
28
Epidermis and Stratum Germinativum. Although there are several possible sites for vaccine delivery,
including the muscle and mucosal surfaces, it seems that the optimal delivery of vaccines is via
injections into the Epidermis or the Dermis [66]. This kind of injection is difficult to administer with
standard hypodermic needles since the targeted skin thickness is shallow, often below the diameter of
the actual needle and therefore prone to human error.
There is a pronounced public interest in a painfree drug injection device replacing standard
injection needles/syringes [63, 64, 65]. La raison d’être for this interest can be found in the often
traumatic experiences and pain associated with hypodermic needles.
Future work includes the detailed investigation of the mechanisms of liquid injection into the skin
via side opened microneedle arrays. This study should not only include the performance of the actual
micromachined interface at a specific location on the body, but also the influence of the skin on
different locations on the body. If the intra- and/or transdermal liquid delivery proves to be
reproducible, a first step towards an applicable novel instrument will have been taken.
6.2 Dosing & Actuation Unit
6.2.1 Objectives
The dosing and actuation unit (DAU) relieves the patient from concerns about the dosing and
delivery of a drug over a certain period of time. The DAU should thus release the drug without human
interaction. There are two main objectives:
(i) measure the exact quantity of the therapeutic agent and
(ii) pump that quantity through the MTI into the tissue.
The pumping action is also referred to as actuation. In view of prolonged delivery schemes and
excellent portability it is desirable to have an actuator which spares energy consumption and which
has small dimensions. It is very important that the drug is not modified chemically or physically by
the materials used or the actuation principle.
TABLE 6.2:
LIST OF THE MAIN OBJECTIVES OF THE DOSING AND ACTUATION UNIT IN DECREASING ORDER OF PRIORITY.
1. Digital dosing and actuation of the drug to be delivered
2. No chemical modification of the drug
3. Minimize device complexity
4. Minimize dimensions and weight
5. Disposability
Microfluidic Transdermal Liquid Delivery System
29
6.2.2 Expandable Microspheres
Expancel microspheres are produced by Expancel, Sundsvall, Sweden, in a wide variety of types.
They are small plastic particles that expand when heat is applied. The dramatic expansion of the
microspheres when heated is caused by a small amount of a liquid hydrocarbon, e.g. isobutane or
isopentane, encapsulated by a gas tight thermoplastic shell. The shell is composed of a copolymer of
some monomers, e.g. vinylidene chloride, acrylonitrile, and methylmethacrylate. Upon heating, the
shell softens and the liquid undergoes a phase change to gas. Thus, the microsphere is literally
inflated, much like a balloon. This results in a decrease of the density from 1000 to 30 g/l.
After cooling down, the microspheres do not shrink because the thermoplastic shell hardens again.
Thus, the dramatic volume increase is irreversible.
The expandable microspheres are available with expansion temperatures in the range of 70 – 200
°C, whereby the volume increases with the temperature. If the temperature is increased above that at
which the highest expansion is obtained the microspheres will collapse.
The expandable microspheres can be used in contact with many chemicals, including solvents,
without negative effects on expansion. The available size range of unexpanded microspheres is from
5 – 90 µm.
Expancel beads are used in many different applications in the macro scale. For example, in
printing inks to form 3D patterns on paper, for the density decrease of plastics through a controlled
and predictable foaming process, and in reaction injection molding to counteract the shrinkage of
polyurethane.
6.2.3 Technology Demonstrator
There are two fundamentally different ways of measuring an amount of drug that is delivered, for
example, into dermal tissue:
(i) The first is to measure the flow rate of the drug (or parameters directly related to the flow
rate) during the delivery. Integrating the flow rate over time yields the total amount of
drug released into the tissue. This delivery scheme can be defined as being analog, i.e.
knowing rate and time it is theoretically possible to delivery any drug volume.
(ii) The second is to precisely define a number of unit drug volumes during fabrication (e.g.
via precisely fabricated liquid containers). A drug dose can then be achieved by adding up
a multitude of unit drug shots. In a shot, the whole liquid volume enclosed in the container
must be pressed out of the container, much like a microscaled syringe. This scheme is
somewhat digital and represented in Figure 6.6.
Micromachined Interfaces for Medical and Biochemical Applications
30
The analog method requires closed-loop sensing and processing of data to dose the liquid precisely
[67]. A digital system does not require data processing during delivery of a certain chemical dose.
This is an advantage when striving for low complexity of the device. This kind of actuation is
unidirectional and the actuator can be a one-shot device.
In this section the first tests of a novel actuator having the potential to be used in a digital dosing
and actuation unit are shown. This technology demonstrator does not address all of the above-
described objectives and functions but represents a first step to such a device.
The basic idea of the digital DAU is to replace the liquid stored in a container by solid matter as the
liquid is pressed out of the container. Expandable microspheres have the potential to fulfill this
function, as schematically shown in Figure 6.7. Bead expansion is triggered by a thermal signal.
Micromachined test devices were fabricated to demonstrate this concept [Paper 3]. In a one-shot
pump experiment, Expancel microspheres packed against a filter in a microchannel, immersed in
water were heated from 23 °C (i.e. the measured room temperature) to 70 °C under an applied counter
pressure of 100 kPa. After the beads were cooled down to room temperature again, the pressure was
varied between 20 kPa and 100 kPa whereby no volume variation of the microspheres was observed.
This shows that the microspheres expand regardless of a counter pressure of 100 kPa. It also means
that once the liquid to be pumped is displaced, it will not be possible for the liquid to flow back into
the location where it was stored before the actuation, regardless of 100 kPa of counter pressure and
cooling of the microspheres. The somewhat arbitrary value of 100 kPa is imposed by the performance
of the test set-up.
Liquid to bedelivered
Act
uato
r
Actuator
Storage
Delivery
Figure 6.6: Principle of a digital dosing system.
One-Shot Pump
Heat
pp
Liquid
Figure 6.7: Concept of a one-shot
pump based on expandable
microspheres
Microfluidic Transdermal Liquid Delivery System
31
Figure 6.8: Expansion of expandable microspheres packed in a channel and immersed in water.
The expansion process is dependend on the geometry of the bead pack. Provided the depth of the
test channel is constant, the bead pack can be characterized by the ratio L/W, where L is the length of
the pack and W the width of the channel. It was observed that the relative volume expansion is
inversely proportional to the pack ratio L/W. Only the beads furthest away from the filter effectively
contribute to the volume expansion and the rest of the beads literally block the channel. Therefore, to
use the full potential of expandable microspheres, the L/W ratio of bead packs should be as small as
possible [Paper 3].
The DAU presented in this thesis differs from earlier shown discrete drug dosing systems as shown
earlier [68] where the drug is not pumped into the tissue but presented to the tissue via a normally
closed valve.
Other conventional micropumps based on, for example, thermo pneumatic, piezoelectric,
electrostatic, or electromagnetic actuators, require a relatively complex assembly which limits their
use in disposable devices [69, 70, 71]. Direct electrical actuation of liquids via electroosmotic flow
and dielectrophoresis is less suited for drug delivery applications because of the high voltages required
during actuation [72].
An innovative, low complexity, unidirectional liquid actuation concept was recently presented and
is based on controlled evaporation [73]. This principle has the great advantage of not requiring any
electrical actuation.
A dosing system based on the formation of gas bubbles via electrochemical processes during
electrolysis was presented earlier and was designed to dose nanoliter amounts of liquid [67]. The
application field of this device is the monitoring of clinically important substances via microdialysis.
There are two features of the DAU based on expandable microspheres that make it interesting for
transdermal drug delivery:
(i) high energy density, which allows the actuator to overcome high counterpressures
(ii) the fact that the delivered liquid cannot flow back into the DAU after actuation since the
liquid container is filled with expanded beads.
However, challenges remain, including the wafer level localization of the beads [74], the wafer
level integration of the DAU to the MTI, investigation of the thermal compatibility with drugs as well
as sterilizeability.
33
7 Micromachined Electrospray Emitter Tips
for Mass Spectrometry
Electrospray is a method by which ions, present in a solution, can be transferred to the gas phase.
In gas phase, the ions are then accessible to a mass spectrometer that detects their mass to charge ratio.
In this chapter the basic mechanisms of electrospray ionization are explored as well as a
biochemical application area, that is proteomics, where electrospray ionization mass spectrometry is of
great interest. Subsequently, we present an interface, which is designed to transfer biopolymers
present in a solution to gas phase for subsequent mass spectrometry detection.
7.1 Electrospray Ionization
Electrospray is an atmospheric pressure ionization method. The process is known since the
beginning of the century but in the 1960´s the first large molecules were transferred to gas phase [75].
The process involves an electric field concentrated at the apex of a hollow tip containing a solution of
the ions to be transferred to gas phase, as illustrated in Figure 7.1. The tip is often referred to as an
emitter. If the counter electrode generating the electric field has a negative potential, positive ions
accumulate at the emitter orifice. The high density of positive charges at the needle tip leads to the
formation of a Taylor cone [76]. Under a high electric field rE , the repulsive forces become stronger
than the surface tension at the tip of the cone and a liquid spray is generated, i.e. small droplets of
liquid are ejected from the cone.
There are three major steps that extract gas-phase ions from the droplets [77]:
(i) production of charged droplets at the emitter orifice
(ii) shrinkage of the charged droplets by solvent evaporation and repeated droplet
“explosion” due to charge concentration at the surface of the droplet
(iii) the actual mechanism by which the gas-phase ions are produced from the very small and
highly charged droplets.
The third step is not yet fully understood, but two mechanisms have been proposed in literature.
The charge-residue model states that the solvent evaporates and droplets break up until droplets with
only a single ion are formed [75]. The direct emission model suggests that droplets with a radius of
less than 10 nm can allow field desorption, i.e. they directly emit the gaseous ion of interest [78].
Micromachined Interfaces for Medical and Biochemical Applications
34
The opposing effects of the electrostatic force and surface tension of the liquid at the emitter orifice
were investigated [76, 79]. It was found that in the static case, i.e. before droplets are emitted, the
liquid surface takes the shape of a cone with a half angle of 49.3° at the apex. In the dynamic case, the
shape of the cone depends on the experimental conditions.
V+ -
++++
+++++
+
+++++
++++
+ ++++
+++ ++++++ ++ +++
++++++
++
+ +++
+
+ ++
++
+
+
+
+
+
++
+
++
+
+ +
+
+
++
+
+
+
+
+
+ +
+
+
+
--
--
-
-
-
--
-- -
--
---
--
-
-
--
-- --
---
++++
+
+
+
--
-
--
-
E
E
High Voltage
++++
++
-
--
--
rC
Emitter Tip
Solution
CathodeAnode
Cation
Anion
Figure 7.1: Schematic representation of the electrospray ionization process.
7.2 Proteomics
The term “proteome” refers to the total set of proteins expressed in a given cell at a given time; the
study of which is termed proteomics [80]. In the last few years mass spectrometry applied to
biological samples has seen tremendous activity and has become a wide spread technique within
proteomics [81]. Proteomics is complementary to genomics and now includes protein identification,
post-translational modifications, protein function, protein-protein interactions, etc. With the
accumulation of vast amounts of DNA sequences in databases, researchers are realizing that merely
having complete sequences of genomes is not sufficient to clarify biological function [81].
The promise and potential benefits of proteomics are such that corporate and academic researchers
are flocking the area. Apart from purely scientific interest, the disclosure of how protein cascades
change resulting from disorders or drug action has tremendous commercial potential for
pharmaceutical companies [80]. Proteomics directly contributes to drug development since almost all
drugs are directed against proteins.
The success of mass spectrometry (MS) within proteomics is a direct consequence of the
introduction of the soft ionization methods of electrospray ionization [82] and matrix-assisted laser
Micromachined Electrospray Emitter Tips for Mass Spectrometry
35
desorption/ionization (MALDI) [83]. Often, MS relies on the digestion of gel-separated proteins into
peptides by a sequence specific protease such as trypsin. The reason for analyzing peptides rather than
proteins is that the molecular weight of proteins is not usually sufficient for database identification
[81]. In contrast, peptides are easily eluted from gels and even small sets of peptides from a protein
provide sufficient information for identification.
In proteomics research there is a need for large scale, high throughput methods to support the
staggering number of protein analyses [84].
7.3 Micromachined Electrospray Emitter Tips
7.3.1 Objectives
The most obvious goal in investigating micromachined electrospray emitter tips is to create two-
dimensional arrays of emitters along with microfluidic functionality on the same chip. These two
features give the possibility to speed up protein/peptide detection and identification by automating
sample separation, preparation and ionization for mass spectrometry detection. In addition to speeding
up the detection process it also reduces sample volumes and reduces exposure of the analyte to the lab
environment.
The emitter tip itself should show very good spray performance, i.e. a high yield of conversion
from ions in their dissolved state to the gas phase. A high yield can be obtained by low sample flow
rates in the emitter [77]. Low flow rates create small droplets from which ions are extracted faster and
easier than from large droplets. Low flow rates are achieved by low potential differences between the
emitter and the counter electrode [79]. The required potential difference to generate a flow (for given
conditions such as the distance between emitter and counter electrode, physical liquid properties, etc.)
is dependent on how effective the electric field lines are concentrated at the emitter tip and thus
dependent on the base radius rC of the Taylor cone (Figure 7.1). Therefore, the lower rC , the lower the
potential difference required and the better the spray performance.
The differences in dimensions, e.g. orifice diameter or emitter tip length, should be minimized
when comparing tips of the same array and between different arrays, thereby avoiding variations of
spray conditions. Geometrical reproducibility is the key to automated, high throughput electrospray
mass spectrometry.
Table 7.1 is a summary of the objectives of the electrospray emitter work.
7.3.2 Technology Demonstrator
Process technology as presented earlier in chapter 3 was used to manufacture chip based
electrospray emitter tips. The out-of plane tips were arranged in a 2D array as illustrated in Figure 7.2.
Micromachined Interfaces for Medical and Biochemical Applications
36
Two types of emitters were fabricated and self-aligning principles were applied for the fabrication
of the emitter bore. The first type consists of silicon and is referred to as Si-tip; the second type
consists of silicon dioxide and is referred to as SiO2-tip, as depicted in Figure 7.3. The rim of the Si-
tip opening is extremely sharp, in the nanometer range. The circular rim of the SiO2-tip is defined by
the thickness of the grown oxide, which in this case is 1.5 µm in thickness. The rim of the tip opening
is less sharp compared to the edge of the silicon version. For characterization, the substrate wafer was
diced into 1×1 mm chips containing one tip and glued to the end of a fused silica capillary. With this
design the chip/capillary-assembly could be operated as a standard nanoelectrospray tip, as shown in
Figure 7.4.
TABLE 7.1:
LIST OF THE MAIN OBJECTIVES OF THE MICROMACHINED ELECTROSPRAY EMITTER TIPS FOR MASS SPECTROMETRY
IN DECREASING ORDER OF PRIORITY.
1. Enable two dimensional arrays of emitter tips for mass spectrometry
2. Enable integration of microfluidic functionality for each emitter tip without creating large
dead liquid volumes
3. Minimize Taylor cone base radius to the radius of the emitter orifice
4. High reproducibility of the emitter dimensions and geometry
Figure 7.2: Array of MS Tips
Micromachined Electrospray Emitter Tips for Mass Spectrometry
37
70 µ
m
70 µ
m
Figure 7.3: Zoom on both types of electrospray emitter tips. (left) Silicon tip. (right) Silicon dioxide tip.
Let us consider a sample solution and an arbitrary emitter tip as shown in Figure 7.5. The liquid
forms a certain contact angle θ C with the tip material depending on both the liquid and surface
properties. The emitter is characterized by the angle β, which defines how the channel geometry is
altered at the orifice. If β ≥ π/2-θC, the orifice at the apex of the emitter acts as a geometrical stop
valve [85]. In the hydrophilic case, θC < 90°, the liquid will fill the channel via capillary forces and
stop at the orifice. To wet the tapered shaft of the tip, the internal pressure ∆p of the liquid must be
increased so as the liquid meniscus whisks out to form θC between the shaft and the liquid. The larger
β, the larger the required pressure to obtain wetting of the shaft.
Let us now compare the tapered emitter presented in this thesis with flat nozzles, as illustrated in
Figure 7.6. In view of the above mentioned, the optimum size of the Taylor cone is achieved when its
base radius rC corresponds to the radius of the channel orifice in the emitter. It becomes clear now that
a sharp rim limits the Taylor cone base to the orifice whereas in the nozzle case the flat top of the
nozzle is wetted, provided that the flat nozzle top is hydrophilic. Thus the size of the cone is
increased, as schematically represented in Figure 7.6.
When visually examining the spray performance of the tapered tip emitters in an electric field it
was confirmed that the cone does not wet the shaft of the tip (Figure 7.7).
Micromachined Interfaces for Medical and Biochemical Applications
38
Sample container
Pressurized air
Metal connector
Metal bar
XYZ-stage
Mass Spectrometer
Inlet
High voltage
Figure 7.4: Operation of the micromachined nESI emitter demonstrator
Liquid
Tip structure
∆p
β
θ
∆p1
β
θ
β
θ
β
θ∆p2
Figure 7.5: Representation of the geometrical stop
valve and the pressure required to wick the fluid
meniscus, ∆p1 > ∆ p2. All surfaces are hydrophilic.
Liquid
Tip structure
Taylor cone
d d d
β=90°β≅160°β≅160°
Figure 7.6: comparison of the presented tapered
emitter tips and flat nozzle emitter. Because the angle
β is smaller for the nozzle case compared to the
presented sharp rim orifices, the flat nozzle top is
wetted, thus increasing the Taylor cone size. All
surfaces are hydrophilic.
Micromachined Electrospray Emitter Tips for Mass Spectrometry
39
Figure 7.7: Photograph of an electrospray of acetonitrile with 1% acetic acid from a silicon dioxide tip.
The presented micromachined electrospray emitter tips were extensively tested at Analytical
Chemistry, Royal Institute of Technology, Stockholm, Sweden and were successfully utilized for
protein and peptide analysis. A stable Taylor cone was observed during mass spectrometry and most
importantly, a higher repeatability compared to commercial pulled silica capillary tips resulted [86].
High repeatability resulted from the very reproducible emitter dimensions and is prerequisite for
automated, reproducible and robust operation in a high throughput analysis system. Commercial
players (Advion Inc., Ithaca, NY, USA) have recognized the need for such a system and started
commercializing micromachined emitter arrays [6].
Future work can include the integration of chemical functions on the chip containing the
micromachined emitter (array). For example, sample preparation, separation, or concentration.
41
8 Final Conclusions
Deep reactive ion etching is a powerful tool to sculpture truly three-dimensional structures; in
particular differently shaped tapered out-of-plane projections in the micro-scale (i.e. microneedles,
microspikes or microtips).
Medical and biochemical interfaces based on such microprojections show novel, beneficial
features.
The spiked EEG electrode, as an element of the Datex-Ohmeda anesthesia monitor, makes more
efficient surgical procedures plausible without compromising patients comfort and ease of use for
nurses.
The transdermal drug delivery system, and in particular the side-opened microneedle arrays,
considerably increase patient acceptance for transdermal drug delivery. Comfort for the patient is an
important condition when responding to the increased demand for home treatment and self-injection of
active compounds.
The micromachined electrospray ionization emitter tips combine spray efficiency of conventional
high-performance emitters with the possibility to batch fabricate two dimensional emitter arrays with
integrated microfluidics, for example in the separation of analytes.
Interestingly, the presented interfaces reduce the amount and influence of human interaction to
achieve a certain function when compared to their conventional counterparts. There is no need to
prepare the skin and apply gel prior to biopotential measurements. During the delivery of vaccines that
require shallow intradermal injections there is no risk for erroneous delivery too deep into the
subcutaneous tissue or even the muscle. The very reproducible dimensions of micromachined
electrospray emitter tips make that the automation of mass spectrometry analysis of biopolymers
conceivable.
Further research is required to evaluate the reliability and effectiveness of these concepts; an
important step towards applicable instruments. However, this step presents a worthy challenge with
great implications in accuracy, versatility, ease of use, comfort for the patient, as well as reduced
costs, should it prove successful.
43
9 Acknowledgements
With hesitation I consider this “book” as finished and look back on the time I spent in Stockholm.
It was full of adventure, instructive, exciting, sometimes dark and also very light… and cold.
The work was carried out at the Microsystem Technology Group, Department of Signals, Sensors
and Systems, Royal Institute of Technology, Stockholm, Sweden. Funding was gratefully received
from Datex-Ohmeda.
Without the professional or moral support of many persons, it would not have been possible to
come to this point.
In particular, I would like to express my sincere gratitude to:
My supervisor Göran Stemme for enthusiastic and generous guidance, for always being available,
and for enabling me to present this work in many places.
Pekka Meriläinen of Datex-Ohmeda for initiating our collaboration and for sharing his tremendous
experience and many ideas.
My colleagues and friends in the Microsystem Technology group. Wouter van der Wijngaart for
many stimulating discussions, for non-work related activities of the humpy-bumpy type and for being
“en riktig kompis”; Jessica Melin for forgiving me for always handing over manuscripts to be
proofread too close before the deadline (including this thesis…); Kjell Norén for his clever view on
packaging and supreme skills to produce mechanics and electronics; Sjoerd Haasl for optimizing the
world’s first clean-room walk, for his irrevocably positive thinking and for making work after
midnight less boring; Frank Niklaus for literally bleeding for my research and enduring a skin biopsy;
Hans Sohlström for his help in graphical and mathematical issues, especially the cover picture of this
book.
My ex-colleagues Peter Enoksson who got me started at the Microsystem Technology Group and
issued the most valuable ICP license and Helene Andersson for her different view on all sorts of
“small stuff”.
Micromachined Interfaces for Medical and Biochemical Applications
44
My colleagues at Datex-Ohmeda: Heli Tolvanen-Laakso and Magnus Kåll for working on the
spiked electrode project in Helsinki and for helpful discussions around the device.
Stig Ollmar of Medical Engineering, Karolinska Institute, for sharing his extensive knowledge of
skin impedance and for helping with the decisive first impedance measurements on the spiked
electrode.
Johan Sjödahl of Analytical Chemistry, Royal Institute of Technology, for characterizing the ESI-
tips and for explaining the “who’s who” of all kinds of “strange” molecules.
Peter Georén, Carina Lagergren, Anton Lundqvist and Anna-Karin Hjelm of the Department of
Chemical Engineering Technologies, Royal Institute of Technology, for instructions on how to do
impedance measurements on, hopefully still, healthy volunteers.
Marianne Ekman and Lennart Emtestam for their support in obtaining skin sections.
Martin and Emma for many superb hours together: in a kajak, on a mountain bike or on Norwegian
mountains (cramponer definitely rule!).
Roli and Oli back in Switzerland for being true friends.
My mother, father, sister and brother-in-law: I love you too!
45
1 0 Summary of the Appended Papers
Paper 1
The microfabrication of spike arrays is described. The shown packaging allowed for the preliminary
testing of the arrays as biopotential electrodes.
Electrode-skin-electrode impedance measurements confirmed that spiked electrodes do not need any
skin preparation or electrolytic gel to achieve comparable performance to standard wet electrodes.
Thus, spiked electrodes allow high quality recording of low level biopotentials such as signals
produced by the human brain (i.e. EEG recording). The application of the electrode on and removal
from the skin is fast and uncomplicated. Furthermore, the size of the spiked electrode can significantly
be reduced compared to standard electrodes.
Paper 2
Microfabrication methods for the machining of barbed spikes is described. The barbs are means of
attaching a chip to surfaces capable of being penetrated by silicon spikes. The attachment principle is
inspired by fishhooks and arrows whereby the barbs penetrate a material but offer resistance to
detaching forces.
The mechanical attachment of two types of barbed spike arrays was measured on non-biological
materials (Polyethylene foil, Aluminum foil and ParafilmTM) as well as on human skin. The maximum
adhesion force was 1,36 N achieved by a 2x2 mm2 chip containing 64 barbed spikes, 160 µm in
length, pressed into Polyethylene foil. Maximum adhesion to skin was 0,54 N.
Paper 3
Two novel concepts for controlled handling of liquids in microfluidic systems are presented: a one-
shot micropump and a normally open one-shot valve based on Expancel microspheres.
Expancel® microspheres are small spherical plastic particles that, when heated, increase their volume
considerably. It is shown that liquid volumes in the nanoliter range can be actuated against a counter
pressure of at least 100 kPa and that a channel can block fluid flow in a microchannel against
pressures of at least 100 kPa. The emphasis in relation to this thesis is on the one-shot pump since a
possible application of such a device is in transdermal drug delivery. The pumping principle is largely
Micromachined Interfaces for Medical and Biochemical Applications
46
independent on the liquid properties and prevents the delivered liquid from flowing back into the
pump.
A short review on different types of liquid handling (pumping and valving) is given in the introduction
of this paper.
Paper 4
The characterization of dry spiked biopotential electrodes is presented. It was found that the
electrode-skin-electrode impedance (ESEI) is strongly dependent on the electrode size (i.e. the
number of spikes) and the coating material of the spikes. Electrodes larger than 3x3 mm2 coated with
Ag/AgCl have sufficiently low ESEI to be well suited for electroencephalograph (EEG) recordings.
The maximum measured ESEI was 4.24 kΩ and 87 kΩ, at 1 kHz and 0.6 Hz, respectively. The
minimum ESEI was 0.65 kΩ and 16 kΩ, at the same frequencies. The ESEI of spiked electrodes is
stable over an extended period of time. The arithmetical mean of the generated DC offset voltage of
11.8 mV immediately after application on the skin and 9.8 mV after 20 to 30 min. A spectral study of
the generated potential difference was unstable at frequencies below approximately 0.8 Hz. Thus, the
signal does not interfere with a number of clinical applications using real time EEG.
Comparing raw EEG recordings of the spiked electrode to commercial high performance electrodes
(i.e. ZipprepTM) showed that both signals were similar. Due to the mechanical strength of the silicon
microneedles and the fact that neither skin preparation nor electrolytic gel is required, use of the
spiked electrode is convenient. The spiked electrode is very comfortable for the patient.
Paper 5
The first hollow out-of-wafer-plane silicon microneedles having openings in the shaft rather than
having an orifice at the tip are presented. These structures are well suited for transdermal microfluidic
applications, e.g. drug- or vaccine delivery. The developed deep reactive ion etching (DRIE) process
allows fabrication of two dimensional, mechanically highly resistant needle arrays offering low
resistance to liquid flows and a large liquid delivery area for the tissue. The presented process does
not require much wafer handling and only two photolithography steps are required. Using a 3x3 mm2
chip in a typical application, e.g. vaccine delivery, a 100 µl volume of aqueous fluid injected in 2 s
would cause a pressure drop of less than 2 kPa.
A short review on microprojections in the biomedical field is given.
Paper 6
Two novel types of micromachined nanoelectrospray emitter tips were designed, fabricated and tested.
The fabrication method of the hollow tips is based on a self-aligning deep reactive ion etch (DRIE)
process. The tips consist either of silicon dioxide or silicon and feature orifice diameters of 10 and
18 µm, respectively. The geometrical characteristics of both emitter types are favorable for the
Summary of the Appended
47
generation of stable electrospray ionization, i.e. wetting of the tip shaft is avoided and the base of the
Taylor cone is limited to the diameter of the orifice.
A silicon dioxide tip was operated in a bench top set-up to visually evaluate of the electrospray. Both
kinds of tips were also successfully used for the analysis of an insulin sample in an ion trap mass
spectrometer.
A short review on the use of microengineering in electrospray mass spectrometry is given in the
introduction of this paper.
49
1 1 Glossary
anatomy a branch of biology that deals with the study of the structure of organisms
anesthesia loss of sensation with or without loss of consciousness
anisotropic exhibiting properties (for example etch speed) with different values when
measured along different directions
aspect-ratio ratio of the length to the width of a structure (for example a hole or a
projection)
barb a sharp projection extending backward (as from the point of an arrow or
fishhook) and preventing easy extraction
biopolymer a polymeric substance (as a protein) formed in a biological system
biopotential potential created on the skin or within the body due to the excitation of active
cells such as nerves or muscles
cardiology the study of the heart and its action and diseases
collagen an insoluble fibrous protein that occurs in vertebrates as the chief constituent of
connective tissue fibrils and in bones. It is connective tissue of the dermis that
when properly prepared from animals by tanning, becomes the tough leather
that has many uses.
dendritic cells dendritic cells are located strategically at the interface to potential pathogen
entry sites and take up antigen, move into lymphoid tissues and trigger immune
response
electrocardiography detecting and recording the changes of electrical potential occurring during the
heartbeat. Used especially in diagnosing abnormalities of heart action
electroencephalography detecting and recording brain activity
electromyography detecting and recording activity associated with functioning skeletal muscle
follicle a) a small anatomical cavity or deep narrow-mouthed depression
b) a small lymph node
gauge the diameter of a slender object (as wire or a hypodermic needle)
gland cell, group of cells, or organ that selectively removes materials from the blood,
concentrates or alters them, and secretes them for further use in the body or for
Micromachined Interfaces for Medical and Biochemical Applications
50
elimination from the body
hypnosis any of various conditions that resemble sleep
hypodermic adapted for use in or administered by injection beneath the skin
intradermal situated, occurring, or done within or between the layers of the skin; also :
administered by entering the skin
invasive a medical procedure which requires puncture or incision of the skin or insertion
of an instrument or foreign material into the body.
isotropic exhibiting properties (for example etch speed) with the same values when
measured along all axes
keratin any of various sulfur-containing fibrous proteins that form the chemical basis of
horny epidermal tissues (as hair and nails)
neurology the scientific study of the nervous system especially in respect to its structure,
functions, and abnormalities
parenteral an administration route for drugs via injection. This may be intravenously (into
a vein), subcutaneously (under the skin) or intramuscularly (into a muscle)
pathogen a specific causative agent (as a bacterium or virus) of disease
physiology a branch of biology that deals with the functions and activities of life or of
living matter (as organs, tissues, or cells)
proteomics study of the total set of proteins in a given cell at a given time
subcutaneous being, living, used, or made under the skin
transdermal through the dermis
trauma tissue damage caused by external force
ventilator an adjustable positive pressure generator that acts as an artificial breathing
mechanism during anesthesia or respiratory depression
51
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