A TEMPLATE-BASED FABRICATION TECHNIQUE FOR SPATIALLY-DESIGNED POLYMER MICRO/NANOFIBER COMPOSITES

Nisarga Naik1*, Jeff Caves2, Vivek Kumar2, Elliot Chaikof2, Mark G. Allen1

1School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 2Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta,

GA

ABSTRACT This paper reports a template-based technique for the

fabrication of polymer micro/nanofiber composites, exercising control over the fiber dimensions and alignment. Unlike conventional spinning-based methods of fiber production, the presented approach is based on micro-transfer molding. It is a parallel processing technique capable of producing fibers with control over both in-plane and out-of-plane geometries, in addition to packing density and layout of the fibers. Collagen has been used as a test polymer to demonstrate the concept. Hollow and solid collagen fibers with various spatial layouts have been fabricated. Produced fibers have widths ranging from 2 µm to 50 µm, and fiber thicknesses ranging from 300 nm to 3 µm. Also, three-dimensionality of the process has been demonstrated by producing in-plane serpentine fibers with designed arc lengths, out-of-plane wavy fibers, fibers with focalized particle encapsulation, and porous fibers with desired periodicity and pore sizes.

KEYWORDS

Micro/nanofibers, Microtransfer molding, Collagen, Fiber composite

INTRODUCTION

Over the past few decades, substantial research has been directed towards the fabrication of polymer micro/nanofiber reinforced composites for a variety of applications including clothing, aerospace engineering, and biological applications. One of the prime features offered by a fiber composite is its enhanced mechanical characteristics as compared to the constituent matrix material or the fibers. The mechanical properties of fiber composites are governed not only by the fiber properties and dimensions, but also by the fiber distribution and volume fraction in the matrix. Randomly aligned fibers pose a difficulty in packing large proportions of fibers tightly, also making it harder to predict the mechanical behavior of the composite. A controlled and orderly fiber arrangement enables tunability of the mechanical properties of the resultant composite material. In addition to good fiber alignment, certain applications require control over the shape and spatial layout of the fibers. Although considerable advancements have been made in the currently existing methods of polymer micro/nanofiber production, such as electrospinning [1], wet-spinning [2],

and drawing [3], it is challenging to obtain precise control over the dimensions, alignment and layout of the fibers produced in these fashions within a composite.

The presented method exploits molding techniques to fabricate parallel fibers with desired placement, sizes and shapes. Conventional MEMS fabrication processes are utilized to sculpt and demold the fibers. Also, this process obviates handling of large numbers of fibers for building composites as it involves a simple transfer of the fabricated fibers to the matrix material without disturbing the arrangement.

FABRICATION METHOD

The reported fabrication method is based on microtransfer molding [4], a process used for building free-standing three-dimensional polymer negative replicas of a template. Figure 1 shows a schematic of the fabrication process sequence. Trenches with the required fiber layout design are etched into a silicon wafer using inductively coupled plasma (ICP) etching. This step is optionally followed by a potassium hydroxide (KOH) etching process to sharpen the tips of the trenches. This helps reduce polymer webbing due to surface tension effects, and facilitates the fiber individualization and demolding of fibers. The template is then coated with a thin layer of parylene to aid in fiber release (Figure 1a). A desired volume and concentration of collagen solution is poured on the template and degassed in vacuum.

Figure 1. Fabrication process sequence, a) Deposit parylene on the silicon template, b) Solvent cast collagen solution, c) Spray coat PVP protectant, d) Individualize fibers by dry mechanical grinding or RIE, e) and f) Extract fibers using a water soluble tape after dissolving away PVP.

a)

b)

c)

d)

e)

f)

Parylene

Silicon

Collagen

PVP

Water soluble tape

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This is followed by solvent casting to forcollagen film on the template (FiguresPolyvinylpyrrolidone (PVP), a water soluspray coated on the template. The polymtrenches, selectively masking the collagetroughs. It acts as a protectant for the fibeindividualization (Figure 1c). The portion the upper part of the trenches is remmechanical polishing or reactive ion forming individualized collagen micro/nantrenches (Figure 1d). The fibers are extritrenches using a water-soluble tape with thretained (Figures 1e, 1f and 2c).

Figure 2. SEM images of, a) and b) cross solvent cast collagen film on a silicon temsharpened trenches, c) 25 μm wide indivicollagen fibers extracted from the templaalignment of the fibers is retained, d) 25 μmfibers separated from the template by direct peel HOLLOW AND SOLID FIBER DIME

The wall thickness of the fibers deconcentration, and volume of the collagenfor a given surface area. The solvent castcan be roughly estimated by equation (1).

Td = Tw × M = VS

× Cρ

Where, Td = Thickness of dry cast film Tw = Thickness of wet film M = % of solid collagen in the solution V = Volume of collagen solution used S = Effective surface area of the tem

solvent casting C = Concentration of collagen solution uρ = Density of collagen

c d

Silicon

Collagen fil

a b

50µm

rm a conformal s 1b and 2b). uble polymer is mer fills up the en film in the ers during fiber of the film on

moved by dry etching (RIE), nofibers in the cated from the heir alignments

section view of mplate with tip idualized hollow ate using tape, m wide collagen ling.

ENSIONS epends on the n solution used t film thickness

(1)

mplate used for

used

Figure 3 illustrates the vathickness with the concentrationsolution. The measured values foestimated values. The noted diffbe attributed to losses resultipolymer solution to the bottom effective surface area.

Figure 3. Variation of fiber wasolution concentration for a fixed vthe collagen solution for a concentr

By regulating the dimension

template, and the collagen solutior solid fibers are obtained. Figrectangular cross-section hollow20 μm deep template with 3 ml ocollagen, and a ribbon-like solidμm deep template with 5 ml ocollagen, respectively.

The width and length of the ftemplate design. This method hfibers with widths spanning f(Figure 4c) to 50 µm (length- 4varying from 300 nm to 3 µm.

Figure 4. SEM images of, a) a ribbon like solid collagen fiber, antape.

00.20.40.60.8

11.21.41.6

0 1

Poly

mer

solu

tion

conc

entr

atio

n (m

g/m

l)

Fiber thick

Volume = 4 ml

a b

n template

lm

c

Measured

1mm

25µm

15µm

Hollow fiber lumen

ariation of the fiber wall n and volume of collagen ollow the same trend as the ferences in the values may ing from seepage of the

m of the die, increasing the

all thickness with collagen

volume of 4 ml, and volume of ration of 1.5 mg/ml.

s and cross sections of the ion properties used, hollow

gures 4a and 4b illustrate a w fiber fabricated using a of 1.5 mg/ml concentration d fiber fabricated using a 4 of 2 mg/ml concentration

fiber are determined by the has been used to fabricate from 2 µm (length-1cm) 4cm) and wall thicknesses

hollow collagen fiber, b) a nd c) 2 µm hollow fibers on

01234567

2 3

Poly

mer

solu

tion

volu

me

(ml)

kness (µm)

Concentration = 1.5 mg/ml

b

Estimated

25µm

15µm

Solid fiber

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SPATIALLY DESIGNED FIBER COFor applications demanding three-dime

over the manipulation of the fibers, apprtemplate designs with the desired strhorizontal and vertical planes of the moobtain the required spatial fiber layouts. In-plane and out-of-plane crimped fibers

A certain amount of slack in the loadheld in a matrix material can improve the eresultant composite material. Many soft human body, such as tendons and ligamentof crimped fiber bundles for this reason [5]and out-of-plane wavy fibers can be fabrictemplate-based method. For in-plane untemplates with serpentine trenches are used arc lengths of the serpentine patterns arfabricate in-plane wavy fibers with designedranging from 11 % to 57 %. The templates fwavy fibers are fabricated to have a multi-necessary for delineating the out-of-planethe resultant fibers (Figure 6a). SequentiKOH etching steps are used to fabricate thFor this process, solvent casting of collagreplaced by spray coating over the templaundulated fibers, preseparated from transferred directly to a water soluble tape (F

Figure 5. SEM images of, a) a serpentine silicb) in-plane serpentine collagen fibers with a de57%.

Figure 6. SEM images of, a) a multi-depth silithe fabrication of out-of-plane wavy fibers, andwavy collagen fibers.

a b

a

200 µm

OMPOSITE ensional control ropriate silicon ructure in the ld are used to

d bearing fibers elasticity of the tissues in the

ts, are made up ]. Such in-plane cated using the

ndulated fibers, (Figure 5). The re modified to d failure strains for out-of-plane -depth structure e geometries of ial plasma and hese templates. gen solution is ate. This forms

one another, Figure 6b).

con template, and esigned strain of

icon template for d b) out-of-plane

Fibers with localized particle eOften applications demand en

as drug loaded particles [6] andSilicon templates used in the tebe tailored to fabricate fiberencapsulation. These templatesplaced in the trenches to captureThe well diameter is designed tparticle size. This concept is beads (diameters: 10-30 µm) asfilm is cast over the template. Afthe glass beads are driven into tpattern guided self-assembly (Fof collagen is cast on the temhollow fibers with glass beads enwells (Figure 7c).

Figure 7. SEM images of, a) fabrication of fibers for particle eplaced in periodic wells along holltrenches of the template, and ccollagen fibers extricated using tape Porous fibers

Using templates with pillarfibers are fabricated (Figure placement can be designed as renetworks can find applicability ichemical sensing.

Figure 8. SEM images of, a) a silthe trenches, and b) porous collagen

a

b

ab

100 µm

50 µm

encapsulation ntrapment of material such

d cells [7] in hollow fibers. emplate-based method can rs for focalized particle s have wells periodically e the particles (Figure 7a). to be larger than the target demonstrated using glass

s test particles. A collagen fter fiber individualization, the fiber wells by template

Figure 7b). A second layer mplate, forming enclosed ncapsulated in the periodic

a silicon template for the ncapsulation, b) glass beads low collagen fibers inside the c) glass bead encapsulated e.

rs in the trenches, porous 8). The pore size and

equired. Such porous fiber in the area of filtration and

licon template with pillars in n fibers on tape.

c

Glass bead inside a collagen fiber well

b

50 µm

50 µm

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Fabrication of composite material For the fabrication of fiber reinforced c

matrix material is cast on the water solubthe produced fibers. The tape is dissolvedbehind fibers embedded in the matrix. composite with collagen fibers in biologica(elastin) has been demonstrated. Such a scuse in tissue engineering applications. Figunidirectional aligned collagen microfiblamina.

Figure 9. Microscope image of an aligned collacomposite in elastin matrix. MECHANICAL CHARACTERIZAT

Stress-strain relationship of the fiber derived by applying a constant strain rate until failure. Preliminary mechanical tests smaterial exhibits the expected trend of stiffefiber direction as compared to the directionto it (Figure 10). Young’s modulus of thmaterials along the fiber direction is observtimes the modulus in the perpendicudemonstrating their orthotropic nature (Ttests have been conducted on fiber coapproximately 1% fiber volume fraction. Nof the elastin film have not been taken intomaking these measurements.

Figure 10. Stress-strain relationship of composite illustrating material stiffening adirection as compared to across the fiber directi

020000400006000080000

100000120000140000160000180000

0 5 10

Stre

ss (P

a)

Strain (%)

Along the fiber direction

Across the fiber direction

Al

Al

Ac

Ac

composites, the le tape holding d away leaving

A microfiber al tissue matrix caffold can find gure 9 shows a ber composite

agen microfiber

TION composites is

to the material suggest that the fening along the n perpendicular hese composite ved to be 2.2-3 ular direction,

Table 1). These omposites with on-uniformities

o account while

collagen-elastin along the fiber ion.

Table 1. Young’s moduli of collagealong the fiber direction and calculated between 4-6 % strains.

Direction of the applied load

YoSample-1

Along the fiber orientation

1500

Perpendicular to the fiber orientation

500

CONCLUSION

Fabrication of polymer miusing a microtransfer-moldingbeen established. The fabricationa well defined spatial layout ocontrol over their dimensiocomposite material has been dapplication as a tissue scaffoldextended to other polymers anscaled down to form nano-width ACKNOWLEDGEMENT

We would like to thank Mr. Institute of Technology for his gThis project was partially suppoR01 HL 083867).

REFERENCES [1] Feijen, J.; Buttafoco, L.; Kolkman

Poot, A.A.; Dijkstra, P.J.; Vermes,and elastin for tissue engineering an 5, Feb. 2006, p 724-34

[2] Tatagiri, G.E.; Collins, G.; Jaffe, Mand characterizing thermal, mecproperties”; Proceedings of the Bioengineering Conference, 2004,

[3] Nain, A. S.; Wong, J. C.; Amon, Cpolymer micro-/nanofibers usingPhysics Letters, v 89, n 18, 2006, p

[4] Brehmer, M.; Conrad, L.; Funk, lithography”; Journal of Dispersionn 3-4, May/July, 2003, p 291-304

[5] Freed, A. D.; Doehring, T. C.; “Elafibrils”; Journal of Biomechanical 2005, p 587-593

[6] Polacco, G.; Cascone, M. Grazia; LG.; “Biodegradable hollow fnanoparticles as controlled International, v 51, n 12, Decembe

[7] Kim, B.; Kim, I.; Choi, W.; K“Fabrication of cell-encapsulatedusing microfluidic channel”; Jouand Engineering, v 130, n 2, p 1-6

CONTACT *Nisarga Naik, tel: +1-425-233-9

0

long fiber-1

long fiber-2

cross fiber-1

cross fiber-2

50 µm

n-elastin composite materials across the fiber direction,

oung’s modulus (kPa) Sample-2

900

400

icro/nanofiber composites g-based methodology has n method is shown to offer of the fibers along with a ons. A collagen-elastin demonstrated for potential d. This technique can be

nd also can be potentially h fibers.

Richard Shafer of Georgia great support on the project. orted by NIH (award no. -

n, N.G.; Engbers-Buijtenhuijs, P.; , I.; “Electrospinning of collagen applications”; Biomaterials, v 27,

M.; “Spinning of collagen fibers chanical, tensile and structural

IEEE 30th Annual Northeast p 134-5

C.; Sitti, M.; “Drawing suspended g glass micropipettes”; Applied p 183105 L.; “New developments in soft n Science and Technology, v 24,

astic model for crimped collagen l Engineering, v 127, n 4, August

L., Luigi; F., Stefania; G., Paolo, fibres containing drug-loaded

release systems”; Polymer er, 2002, p 1464-1472 Kim, S. W.; Kim, J.; Lim, G.,

d alginate microfiber scaffold urnal of Manufacturing Science , April 2008

9022; nisarga@gatech.edu

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