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SilkandSiliconbasedDevicesforBio-IntegratedandBio-ResorbableElectronics

Research·May2016

DOI:10.13140/RG.2.1.4280.6647

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DawnJohnMullassery

UniversityofBritishColumbia-Vancouver

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Abstract—This paper focuses on the use of silk and silicon along

with other materials to make bio-compatible electronics. Silk acts

as a platform for electronics to be built on its surface. Both bio-

integrated and transient bio-resorbable electronics can be

developed using these devices - based on the application. For

Human Machine Interfaces (HMI), where constant connection

with the brain surface is required, we use non-transient bio-

integration, to obtain high resolution output. Similar technology

can be used for electrocorticography (brain mapping for seizure

patients). Special fabrication methods causes minimal stresses on

the brain tissue and provides good conformal coverage. Transient

electronics is bio-resorbable electronics, where in the device

dissolves within the body after pre-programmed time frames.

This may be used for cases, where the device need not stay in the

body for long periods of time. After the time frame, these

electronics are resorbed by the body, and completely disappears.

Dissolvable films of silk and silicon are used for transient

electronics. The application is wide and can be used for both bio-

medical and non-biomedical purposes.

Index Terms—Bio-integration, Transient electronics, Silk

Electronics, curvilinear surface electronics, Dissolvable

electronics.

I. INTRODUCTION

UMAN race has come a long way from bulky electronics,

and slow devices to an era of sleek and high performance

electronic devices. Even though so much advancements were

made, some electronic technologies could not be fully utilized

due to unsolvable problems, until a few years back. Although

the field of bio-integration has been developing, we were not

able to make a huge impact on the integration of electronics on

body with ease and efficiency. The main reason is the soft,

elastic and curved surfaces of the body. The integration of

such a biological system with a hard rigid wafer of silicon will

lead to a mismatch in mechanics and geometry [1]. If tackling

this problem can be done effectively, then we may be able to

make progress in bio-electronic integration that was never

possible using conventional rigid silicon electronics. The

existing technologies include Utah electrode arrays that act as

penetrating multi-electrode arrays with dimensions similar to

the cortical neurons, that they target for recording or

stimulation. This technology is able to maintain a stable signal

Report submitted on 14/04/2016

Dawn John Mullassery is a student at The University of British Columbia,

Vancouver, BC, CANADA. (e-mail: dawn.john@alumni.ubc.ca).

in the CNS. But these arrays penetrate and may lead to the

formation of micro-hemorrhages [3]. They also lose their

electrical interface stability due to unwanted biological

responses towards electrodes [4]. Another technology used is

the flexible multi-channel electro corticography electrode

array (ECoG array) [5]. But due to their widely spaced

electrode arrays, and large contact electrodes, the electrical

signals on the brain surface are under-sampled [4]. So, the

ECoG arrays cannot be used where high spatiotemporal map

outputs are needed.

The focus is to make electronics that are bio-compatible,

bendy and curvy, that will act as an effective contact on

uneven surfaces like brain. This can be done using materials

that can bend and form complex and curvilinear shapes

without compromising the performance and bio-compatibility.

Although new materials may be used to achieve this property,

silicon tend to act as perfect substrates for integrated

electronics at reduced thicknesses. When silicon is reduced to

nano-scale, the hard and rigid property of silicon will change,

and silicon becomes a soft and floppy membrane [6]. Building

electronics on a nano-silicon structure, and imprinting other

electronic components can help to obtain a curvy and bendable

form of 'Bio-Integrated electronic platform'.

The future of bio-electronics is bendable and curvy, but not

just limited to that. We can modify and use other materials to

build 'Bio-Resorbable Transient Electronics'. Transient

electronics are electronic devices that physically disappear at

prescribed times and at controlled rates [2]. These transient

electronics can be used for applications in implantable medical

devices, and also for non-biomedical applications that will be

discussed later in the paper.

II. DISCUSSION

It is important to understand the possibility of using materials

to make electronics that are bendable and curvy. If silicon

electronics can be made bendable, then they can be laminated

on the surface of an organ like brain. Thus the electrical

signals can be monitored more effectively. John A Rogers and

his research group have found an effective solution to this, and

is relatively simple. They found that by making silicon sheets

ultrathin, it can be made flexible. The reason is common for

any material (Bending strain is inversely proportional to

thickness) [7]. This is followed by fabricating other electronics

on to the silicon substrate.

Silk and Silicon based Devices for

Bio-Integrated and Bio-Resorbable Electronics

Dawn John Mullassery, Electrical and Computer Engineering, The University of British Columbia

H

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A. Bio-Integrated Electronics

Bio-Integrated Electronics is the possible next best leap in

biomedical field. The process of Bio-Integration will be

possible only if suitable bio-compatible efficient techniques

are available. Bio-Integration can be of two major types.

(i) Epidermal Electronics and (ii) Bio-Integration within the

body.

(i) Epidermal electronics can be implanted on the surface of

the skin (on any epidermal part of the body). Much work is

done on this technology and researchers have made

commendable progress in a span of some years [7]. In this

method they use the similar technique of using silicon nano-

membranes for fabrication. The epidermal electronics is also

made stretchy so as to stretch when the skin stretches. These

devices will have multifunctional sensors (such as

temperature, strain, and electrophysiological), microscale

light-emitting diodes (LEDs) and many other active/passive

circuit elements [8]. They also make use of serpentine

nanoribbons that help in stretching and returning back to its

non-deformed state as shown in Fig 1.

Fig 1 Epidermal Electronics on skin in compressed and stretched form.; Fig taken from [8]

(ii) Bio-Integration within human body is possible using the

same techniques as of epidermal electronics. Here also the

silicon nanomembranes are used as a substrate. In some cases,

materials like polyimide (PI) and polymethylmethacrylate

(PMMA) are put to use as well [4]. If the purpose of the

device is to monitor electrical signals on the surface of the

brain, then electrodes have to be implanted on the surface of

the brain. To get the best conformal contact, the thickness

should be minimal as observed in Fig 2.

In this method, the electrodes are coated on a carrier wafer of

Silicon coated with a sacrificial layer of PMMA. On top of

that a PI layer is also coated. After this, the Gold or Chromium

electrode array is coated using electron beam evaporation [4].

The unwanted PI is removed using etching. PMMI is later

removed, and the whole electrode array is transferred from the

Si/PMMA/PI carrier wafer to a layer of silk film. An

Anisotropic Contact Film (ACF) is connected as an

interconnected between the arrays to the external data

acquisition system and PI is coated over interconnects to

prevent unwanted electrical short circuit. The device

fabricated is placed on the brain surface, and the silk film is

slowly dissolves using saline. As the silk is removed, the

electrode array mesh (<10µm) gets high degree of conformal

contact due to its reduced thickness as seen in Fig 2.

Fig 2. The conformal contact increases with reduced thickness.; Fig taken

from [4]

The observations were confirmed using experimental studies

on a feline brain, and the spatiotemporal output obtained

showed excellent results.

Other Bio-Integration techniques include similar approaches

on other internal organs. An electrophysiology mapping

device was also created and implanted on a porcine heart [9].

In this case the heart is a constantly moving surface, and the

substrate should be strong enough to hold the strain. For that

reason, a layer of thin plastic sheet is used as a physical

platform. The crystal silicon nanoribbons are fabricated on the

silicon wafer and transfer printed to the polyimide plastic

sheet. Unlike previous brain mapping system using simple

electrodes, this electrophysiology device uses more

complicated electronics, including amplifier and multiplexing

transistor. The transistors are fabricated using proper

chemical-vapor deposition method, photolithography and

etching. The gold metal act as electrodes for the device. Fig

3(A) shows a final device structure ready to implanted on to

the epicardial surface.

The device fabricated was placed on the epicardial surface.

The mapping device adhered on to the surface due to the

conformal wrapping and adhesion energy at reduced

thicknesses as seen in Fig 3(B). Data obtained is processed

using custom MATLAB software. By monitoring the data

obtained, the propagation of paced and unpaced cardiac

depolarization wavefronts are determined [9].

Fig 3 (A) An electrophysiology mapping device in bent state. (B) Device conforming to the epicardial tissue due to the surface tension and

adhesion forces. Both figures are taken from [9].

In another method an even improved methodology to measure

the spatiotemporal cardiac measurements were taken. This

method used 3D elastic membranes shaped to match the

epicardium of the heart via the use of 3D printing. It also had

A B

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a platform for deformable arrays of multifunctional sensors,

electronic and optoelectronic components [10].

The future of Bio-Integrated electronics is bright and

researchers are trying to improve the properties even more.

Another advancement in this area of research was also

developed when dissolvable electronics were developed using

Silk and Silicon membranes, discussed below.

B. Transient Electronics

Transient electronics are electronic devices that can dissolve

and physically disappear in preset time frames. By placing

these transient electronic devices within the body, they can be

used for many biomedical applications for a medically useful

time frame, and then removed without surgery or other

incisions.

The physics behind this method lies with the corrosion

physics, where in Silicon membranes dissolve readily in water

[11]. The Fig 4(A) shows the dissolution of a silk membrane

in phosphate buffer solution (PBS) over a period of days.

Fig 4(A) Atomic force microscope topographical images of a Si NM (initial

dimensions: 3 mm×3 mm× 70 nm) at various stages of hydrolysis in PBS at 37°C (B) Transient device that includes transistors, diodes, inductors,

capacitors, and resistors, with interconnects and interlayer dielectrics, all on a

thin silk substrate. (C) Experimental (symbols) and theoretical (lines) results for time-dependent dissolution of Si NMs (35 nm, black; 70 nm, blue; 100

nm, red) in PBS at 37°C. Figures taken from [11].

Fig 4(C) shows the rate of dissolution with respect to the

thickness. Three Silicon NanoMembranes of different

thicknesses are dissolved and the rate of dissolution increases

with decreased thickness. Hence the timeframe of the device

can be determined and programmed by controlling the

thickness.

Silk acts as a wafer substrate on which the silicon electronics

and other devices are built upon. Fig 4 (B) shows a final

device structure that is ready to be used. It consists of

resistors, diodes, capacitors, transistors and inductors. In some

other applications where high performance transistors, diodes,

photo detectors, solar cells, temperature sensors, strain gauges,

and other semiconductor devices are needed, silicon dioxide is

used along with silicon.

In the figure 4 (B), the electronic devices include Magnesium

for the conductors, Magnesium Oxide or Silicon Dioxide for

the dielectrics, the Silicon Nanomembranes (NMs) for the

semiconductors and silk for the substrate and packaging

material [11]. Solution casting (Silk), physical vapor

deposition (Mg, MgO, and SiO2) and transfer printing (Si

NMs) are the different techniques that are used for the

fabrication of this device. A layer of protective encapsulation

protects the electronic devices n the circuit. In this case, the

protective film is made using silk (same as the platform). The

devices will only start corroding or dissolving after the

protective layer is fully dissolved. By that period, the required

timeframe for the biomedical application will be met. All of

the components, including the inductors, capacitors, resistors,

diodes, junction transistors, conductors, and interconnects

along with the substrate and encapsulation, will corrode and

dissolve when immersed in deionized (DI) water as shown in

Fig 5.

Fig 5 Images showing the time sequence of dissolution in DI water. Fig taken

from [11]

The chemistry involved in the dissolution process can be

explained using the following equations

Silicon [ Si + 4H2O Si (OH)4 + H2 ]

Silicon Dioxide [ SiO2 + 2H2O Si (OH)4 ]

Magnesium [ Mg + 2H2O Mg (OH)2 + H2 ]

Magnesium Oxide [ MgO + 2H2O Mg (OH)2 ]

All the above equations show the corrosion chemistry [11].

The electrical properties of transient electronic components,

integrated circuits, and sensors are compared with

conventional silicon substrate devices, and their electrical

properties match the conventional devices. Power scavenging

devices like Ultrathin Si solar cells (~3 mm thick) provide fill

factors of 66% and overall power conversion efficiencies of

~3%, even without proper device enhancements like

reflectors, or antireflection coatings [11]. These solar cells

may be considered as an option for power source. Inductors

and capacitors can be used as wireless antennas for mutual

inductance coupling to separately power the external primary

coils.

The scope for the transient electronics is huge. Transient

electronics has the potential to be effective in biomedical as

well as non-biomedical applications. These will be discussed

in the following section.

C. Applications

The applications of the bio-integrated electronics include both

epidermal electronics and for integration within the body. The

A

B

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main focus of the epidermal electronics is to remove heavy

and complicated electronics structures, and to make

electronics simple and easy to work with. Existing epidermal

electronics (Example: constant health monitoring systems) are

inconvenient to carry around and have to be fixed on the body

using straps or bands.

By replacing this method with the epidermal electronics, we

can integrate efficient health monitoring systems on the

surface without complicated systems. MC10 is a company that

works in partnership with Rogers and team to build epidermal

electronics for sports related applications. One such

application include these electronic devices for football or

other games where there is a possibility of concussion. These

devices monitor the impact of the hit during a game, and may

also be used for monitoring heart rate. Similar devices may be

used in hospitals thereby replacing all existing bulky and

complicated systems. Since these are light and less

complicated systems, they can also be used to monitor infants

who are under observation.

When bio-integration within the body is considered, the

impact is even more. Existing devices like Utah electrode

arrays that are ineffective for long term use, and

electrocorticography method that has low resolution output

can be replaced by this technology. As mentioned earlier, it is

effective for all surfaces including heart that is dynamic and

constantly beating. Due to the surface adhesive forces, and

low bending stiffness, these devices will maintain intimate

dynamic interfaces with the body for monitoring electrical

signals from brain surface, and also equally effective in

monitoring heart performance and diagnosing lethal heart

diseases.

Transient electronics is highly useful in the biomedical field,

but not just limited to that. They also hold huge potential in

consumer electronics as well as environmental sensors. One of

the biomedical applications include thermal therapy to control

surgical site infections[11]. These are made using inductive

coils of Mg combined with resistive micro heaters of silicon

nanomembranes. After the required timeframe (3 weeks to 4

weeks) in this case, the transient electronics disappear and will

leave no residue. A similar example is given below, tested on

a mouse.

Fig 6 Images of an implanted (left) and sutured (middle) demonstration

platform for transient electronics located in the

subdermal dorsal region of a mouse. Implant site after 3 weeks(middle). Fig taken from [11].

Fig 6 show a transient electronic device implanted

subdermally in a mouse. After the time frame (3 weeks in this

case), the transient device is fully dissolved. In this way, the

surgical site infections can be avoided relatively easy and

quick. Similarly, they can be used for drug delivery, where in

it delivers drug to a specific spot for a period of time, and after

that it dissolves within the body, thereby removing the

possibility of another surgery to take out the device implanted.

Diseases that are caused due to a nerve bundle or infection in

particular are is ideal for this technology. For instance, the

migraines, cluster headaches and other chronic pains that

happen due to Sphenopalatine Ganglion can be cured using

this technique. A small transient device can be implanted near

the nerve bundle. When the migraine or chronic headaches

occur, the patient can easily an electric charge that will

stimulate the nerve bundle. This can be done using a remote

device. Since these conditions are not long term, the transient

implants can be set for a timeframe of some few years, and

then they slowly resorb in the body.

The non-biomedical applications include using these

electronics in electronic gadgets. The commercially available

gadgets including mobile phones, and computers take more

than desired time to be destroyed, and sometimes they are

thrown away as e-wastes. But we can reduce the problem of

e-wastes by making use of transient electronics. The electronic

devices will be made using transient electronics, and after the

desired period of time, they dissolve away not leaving any

e-wastes behind. Transient electronics can also be used for

'fieldable' environmental sensors that dissolve to eliminate the

need for their retrieval [11].

D. Future

Both bio-integrated and transient electronics have a bright

future. Beyond Biology, stretchable electronics have a future

in engineering industry too. By using stretchable and bendable

electronics, we can make stretchable sensor tapes that can be

wound around big machines to monitor the mechanical and

structural health [1]. In heavy machines that produce high

amount of heat, power scavenging devices can be wound

around, thereby harvesting thermal energy to electrical output.

The future of commercial electronics using transient devices

are also to be developed. Although these devices hold great

potential, developing such a system that can compete with the

existing technology, is an area where much research is needed.

The future for bio-integration is extremely hopeful. By making

more efficient HMIs, the artificial mechanical systems can be

integrated with the biotic system.

The power source is still a matter of concern because of the

losses in wireless charging, or the impracticality in using a

~3% Power Conversion Efficient solar cell as discussed

earlier. If a better power source can be made available, then

the opportunity of using such systems for long-term biological

uses are high.

Future advancements also include the applications on human

body. No trials have yet been done on human body although

much work has been done on other living beings. The only

bio-integration that has been experimented on humans are the

epidermal electronics. Although these devices are ready to

use, they may need more research and development to

incorporate more electronic devices.

These applications and the positive impact this technology can

make on the society, provide strong motivation for continued

and expanded efforts in this emerging field.

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III. CONCLUSION

Clearly, all the scientific advancements discussed in the paper

provide a better future for stretchable, bendable electronics

integrated with biology. The main advancement in the field of

bio-integrated electronics was to understand the basic bending

physics of materials. By reducing the thickness of silicon, the

geometrical mismatch between the biotic surface and silicon

can be removed. Much engineering and research needs to be

done to effectively fabricate the different electronic

components including resistors, capacitors, inductors and

transistors. This can be achieved through proper fabrication

methodologies. Material science and corrosion engineering

have to be considered in the case of transient electronics for

better results.

The results show that there is high hope for HMI integration,

and health monitoring (both epidermal and within the body).

By fabricating efficient electronic devices, we can even

control and monitor the different organs. This will help us to

diagnose any possibility for chronic or fatal diseases.

Stretchable and bio-integrated electronics will soon be a

substitute for the existing conventional methods that are both

invasive and short-term solutions (Utah) and for those that

have low resolution output (current electrocorticography). By

making use of silk and its transient features, the bio-

integration can be made transient also. In the case of transient

electronics, the properties are well dependent on the materials

used. As a result, future research to improve material qualities

would be desirable.

The future applications are not just bio-medical but also for

consumer electronics. Current research advancements are done

by John A Rogers and his group along with other researchers,

and they are trying to integrate this system for effective bio-

medical applications. The application potentials of wearable

and implantable bio-integrated electronic systems needs the

development of more mechanically compatible and

electronically sufficient microcontrollers, memory, power

supply, and wireless data transmission modules. This has also

laid path for future advancements in biotic-abiotic interfaces

and for better and more effective solutions to make biomedical

advancements on biological curvilinear surfaces more easy.

After much needed research and development, this technology

holds the potential to be the next best bio-medical

advancement.

IV. ACKNOWLEDGMENT

I would like to thank Dr. Karen Cheung, The University of

British Columbia, for all the help and guidance in completing

this paper.

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