F

Ya

b

a

ARAA

KLEM

1

oraientWuigtnesLpmab

am

EU

0d

Applied Surface Science 257 (2011) 2432–2435

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

emtosecond laser machining of electrospun membranes

iquan Wua,∗, A.Y. Vorobyevb, Robert L. Clarka, Chunlei Guob

Department of Mechanical Engineering, University of Rochester, Rochester, NY 14627, USAThe Institute of Optics, University of Rochester, Rochester, NY 14627, USA

r t i c l e i n f o

rticle history:eceived 1 June 2010

a b s t r a c t

We demonstrate that a femtosecond laser can be used to machine arbitrary patterns and pattern arrays

ccepted 29 September 2010vailable online 27 October 2010

eywords:aser machining

into free-standing electrospun polycaprolactone (PCL) membranes. We also examine the influence ofvarious laser irradiation settings on the final microstructure of electrospun membranes. A beam fluenceof 0.6 J/cm2 is used to ablate holes in 100 �m thick PCL membranes. The machined holes have an averagediameter of 436 �m and a center-to-center spacing of 1000 �m. Based on these results, the femtosecondablation of electrospun membranes shows great potential for fabricating a variety of functional tissuescaffolds. This technique will advance scaffold design by providing the ability to rapidly tailor surface

izing

lectrospinningembranes morphology, while minim

. Introduction

Electrospinning is a unique and versatile approach for controlledne-dimensional microfabrication with fiber diameters typicallyanging from nanometers to micrometers [1–4]. Due to its compar-tively low cost, relatively high production rate, and simplicity ofnfrastructure and processing, electrospinning is being investigatedxtensively for various applications in materials processing andanotechnology [5–9]. A typical electrospinning process involveshe use of a high voltage bias to charge the surface of liquid droplets.

hen an applied electric field is sufficiently strong, charges builtp on the surface of the droplets will overcome surface tension

nducing the formation of a liquid jet that accelerates toward arounded collector. As an electrospun jet approaches a collector,he jet experiences a fluid instability stage that leads to thin-ing of the jet and solidification of the fibers [10,11]. Because theffect of fluid instability limits the ability to weave the electro-pun fibers, random-assembled membranes are generally obtained.ayer-by-layer electrospinning has been shown to produce fibrousorous membranes in several materials including metals, poly-ers, ceramics, and composites. Porous membranes have multiple

pplications and are actively used in energy storage, healthcare,

iotechnology, and environmental engineering [5,7–9].

Femtosecond laser machining currently shows much promises a versatile tool for the precise processing of materials withicro- and nano-scale features [12–14]. The unique character-

∗ Corresponding author at: Materials Science Program, Department of Mechanicalngineering, University of Rochester, 215 Hopeman Building, Rochester, NY 14627,SA. Tel.: +1 585 275 4844; fax: +1 585 256 2509.

E-mail address: wuyiquan@me.rochester.edu (Y. Wu).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.09.111

and controlling the deformation of the electrospun fibers.© 2010 Elsevier B.V. All rights reserved.

istics of ultra-short laser pulses enable precise processing andminimize sample damage caused by stress waves, thermal conduc-tion, and melting [15]. Researchers have utilized the femtosecondlaser machining of ceramics, polymers, alloys, metals, semicon-ductors, and glass [16–22] to construct a wide range of devices inthe biomedical, photonic, microelectronic, and microfluidic fields.In this paper, we demonstrate the ability of a high-intensityfemtosecond laser to machine holes and slots in electrospun poly-caprolactone (PCL) membranes. The laser system configurationallows for on-demand machining of arbitrary macro- and micro-patterns into electrospun membranes, providing design flexibilitythat could be utilized to assess the structure, function, and appli-cation of electrospun membranes.

2. Experimental setup

For fabrication of electrospun PCL membranes, a precursorsolution of 10 (w/v%) was prepared by dissolving polycapro-lactone (PCL, Aldrich, MW 80,000) in a solvent mixture ofdichloromethane (Aldrich) and methanol (Aldrich) in a ratioof 8:2 by volume. The solution was loaded in a 10 ml plas-tic syringe (BD company) equipped with a 25 gauge nozzle(McMaster), and was pumped through the nozzle, using a syringepump (Cole–Parmer), at a constant flow rate of 2.0 ml/h. A high-voltage power supply (Acopian) was used to provide a potentialvoltage of 18 kV to the stainless steel nozzle. Electrospun mem-branes were collected on grounded aluminum foil or carbon

sheets.

To create structures in electrospun PCL membranes, we usedan amplified Ti:sapphire femtosecond laser system that consistsof a mode-locked oscillator and a two-stage amplifier. The lasersystem generated 65-fs pulses containing 1.5 mJ per pulse at a

Y. Wu et al. / Applied Surface Science 257 (2011) 2432–2435 2433

m8babftosstmbaDsi

diameter of 0.5 �m. The as-prepared membranes were observed

Fa

Fig. 1. Experimental setup for machining of electrospun PCL membranes.

aximum repetition rate of 1 kHz and central wavelength of00 nm. The experimental setup is shown in Fig. 1. The laseream was horizontally polarized and focused by a thin lens ontosample mounted on a motorized X–Y translation stage. The num-er of laser pulses incident on the sample was controlled by aast electromechanical shutter. A beam splitter directed a frac-ion of the incident beam into a joulemeter so that the energyf the femtosecond pulses could be monitored. A neutral den-ity filter was used to adjust the beam intensity incident on theample plane. A desired pattern can be produced by openinghe shutter and laterally translating the sample using the X-Y

echanized stage. The microstructure of the electrospun mem-ranes before and after laser machining were characterized usingfield-emission scanning electron microscope (FE-SEM, Zeiss-Leo

SM982 model) operated at an accelerating voltage of 5 kV. The

amples were sputter coated with a thin film of gold prior to imag-ng.

ig. 3. SEM micrographs of electrospun membranes irradiated by femtosecond laser at dt a scanning speed of 1 mm/s, and (d) F = 0.75 J/cm2 at a scanning speed of 1 mm/s.

Fig. 2. SEM micrograph of electrospun PCL membrane before femtosecond lasermachining.

3. Results and discussion

Fig. 2 shows a scanning electron microscope (SEM) image of a100 �m thick electrospun membrane. The diameters of the elec-trospun fibers range from 0.4 �m to 0.9 �m, with an average

to contain randomly distributed electrospun fibers. A series ofexperiments were conducted to assess the effect of laser fluenceon structure fabrication in the electrospun PCL membranes. Asshown in Fig. 3(a), using femtosecond laser pulses with a fluence

ifferent processing conditions: (a) F = 0.17 J/cm2, (b) F = 0.75 J/cm2, (c) F = 0.17 J/cm2

2434 Y. Wu et al. / Applied Surface Science 257 (2011) 2432–2435

crogra

ofsebdeDwha0ab2hpc0awmba

ibi

fibers and pores.Fig. 5 shows a SEM image of an array of holes machined in a PCL

electrospun membrane with a fluence of 0.6 J/cm2. The distancebetween the centers of the circular holes is 1000 �m, while the

Fig. 4. High magnification SEM mi

f 0.17 J/cm2 caused the electrospun fibers to melt together andorm pores of varying size. The laser-irradiated zone has a verymooth surface with a spot size of about 360 �m. The laser flu-nce was next increased to determine if continuous holes coulde machined into the electrospun membrane without seriouslyisturbing the surrounding area. Fig. 3(b) shows a 600 �m diam-ter hole that was produced using a laser fluence of 0.75 J/cm2.ue to the Gaussian shape of the laser beam, a modified zoneith an average width of 40 �m can be seen around the machinedole. The scanning stage was next set to a speed of 1 mm/s tossess dynamic pattern transfer using the same laser fluences of.17 J/cm2 and 0.75 J/cm2. The stage was programmed to createrectangular pattern. Fig. 3(c) shows that the electrospun mem-

rane has a smooth melting surface with an average width of50 �m, but that the laser energy is not sufficient to machine aole through the electrospun fiber. As in the static case, pores areresent in the irradiated area of the electrospun membrane indi-ating some degree of fiber melting. Increasing the laser fluence to.75 J/cm2 created a rectangular opening with a width of 500 �m,s shown in Fig. 3(d). There is an affected zone with an averageidth of 60 �m around the edge of the machined slot. The smoothorphology of the laser-machined edge suggests that the mem-

rane was not significantly affected outside the laser irradiation

rea.

The divisions between the electrospun fibers, the laser-rradiated area, and the laser-affected zone can be clearly identifiedy examining the higher magnification SEM images in Fig. 4. These

mages also show the connections between electrospun and irradi-

phs of the regions circled in Fig. 3.

ated fibers and allow for the identification of individual electrospun

Fig. 5. SEM image of an array of holes generated in electrospun membranes byfemtosecond laser pulses at fluence of 0.6 J/cm2.

e Scien

dhbrts

4

mwtscpfrb

A

t0

[[[

[[[[

[

Y. Wu et al. / Applied Surfac

iameter of the holes is approximately 436 �m. The laser-machinedoles are homogenously distributed within the electrospun mem-rane and the unexposed areas of the membrane seem to haveetained their smooth morphology. Our study shows that fem-osecond laser machining is an effective way to produce arbitrarytructures in electrospun membranes.

. Conclusion

In summary, a high-intensity femtosecond laser was used toachine electrospun PCL membranes. Using femtosecond laserriting, circular holes and rectangular slots were precisely posi-

ioned and fabricated within the PCL membranes. The presenttudy demonstrates that femtosecond laser machining is an effi-ient technique to prepare patterning structures in electrospunolymeric membranes. This technique could provide a useful plat-orm for the fabrication of functional tissue scaffolds by allowingesearchers to tailor the surface structure of scaffolds for specificiological responses.

cknowledgements

Authors acknowledge Dr. Nathan Jenness for reading and editinghe manuscript. This work was supported with NSF Grants (CMMI609265 and DGE 0221632) and AFOSR (FA9550-10-1-0067).

[[

[[[

ce 257 (2011) 2432–2435 2435

References

[1] Z.M. Huang, Y.Z. Zhang, M. Kotaki, et al., Composite Science & Technology 63(2003) 2223.

[2] A. Frenot, I.S. Chronakis, Current Opinion in Colloid & Interface Science 8 (2003)64.

[3] A. Greiner, J.H. Wendorff, Angewandte Chemie International Edition 46 (2007)5670.

[4] Y.Q. Wu, L.A. Carnell, R.L. Clark, Polymer 48 (2007) 5653.[5] S. Ramakrishna, K. Fujihara, W.E. Teo, et al., Materials Today 9 (2006) 40.[6] Q.P. Pham, U. Sharma, A.G. Mikos, Tissue Engineering 12 (2006) 1197.[7] D. Li, Y.N. Xia, Advanced Materials 16 (2004) 1151.[8] L.S. Nair, S. Bhattacharyya, C.T. Laurencin, Expert Opinion on Biological Therapy

4 (2004) 659.[9] V. Thavasi, G. Singh, S. Ramakrishna, Energy & Environment Science 1 (2008)

205.10] Y.M. Shin, M.M. Hohman, M.P. Brenner, et al., Polymer 42 (2001) 9955.11] M.M. Hohman, M. Shin, G. Rutledge, et al., Physics of Fluids 13 (2001) 2201.12] C.K. Malek, L. Robert, R. Salut, European Physical Journal: Applied Physics 46

(2009) 12503.13] J. Kruger, W. Kautek, Laser Physics 9 (1999) 30.14] M.D. Shirk, P.A. Molian, Journal of Laser Applications 10 (1998) 18.15] Y.V. White, X.X. Li, Z. Sikorski, et al., Optics Express 16 (2008) 14411.16] S. Gaspard, M. Forster, C. Huber, et al., Physical Chemistry Chemical Physics 10

(2008) 6174.17] I.B. Sohn, Y.C. Noh, S.C. Choi, et al., Applied Surface Science 254 (2008) 4919.

18] H. Huang, H.Y. Zheng, Y. Liu, Smart Materials & Structures 14 (2005) S297.19] D.K. Das, T.M. Pollock, Journal of Materials Processing Technology 209 (2009)

5661.20] R. An, J.D. Uram, E.C. Yusko, et al., Optics Letters 33 (2008) 1153.21] A.Y. Vorobyev, C. Guo, Applied Physics Letters 94 (2009) 224102.22] A.Y. Vorobyev, V.S. Makin, C. Guo, Physical Review Letters 102 (2009) 234301.