Femtosecond laser machining of electrospun membranes
Applied Surface Science 257 (2011) 2432–2435
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Femtosecond laser machining of electrospun membranes Yiquan Wu a,∗ , A.Y. Vorobyev b , Robert L. Clark a , Chunlei Guo b a b
Department of Mechanical Engineering, University of Rochester, Rochester, NY 14627, USA The Institute of Optics, University of Rochester, Rochester, NY 14627, USA
a r t i c l e
i n f o
Article history: Received 1 June 2010 Accepted 29 September 2010 Available online 27 October 2010 Keywords: Laser machining Electrospinning Membranes
a b s t r a c t We demonstrate that a femtosecond laser can be used to machine arbitrary patterns and pattern arrays into free-standing electrospun polycaprolactone (PCL) membranes. We also examine the influence of various laser irradiation settings on the final microstructure of electrospun membranes. A beam fluence of 0.6 J/cm2 is used to ablate holes in 100 m thick PCL membranes. The machined holes have an average diameter of 436 m and a center-to-center spacing of 1000 m. Based on these results, the femtosecond ablation of electrospun membranes shows great potential for fabricating a variety of functional tissue scaffolds. This technique will advance scaffold design by providing the ability to rapidly tailor surface morphology, while minimizing and controlling the deformation of the electrospun fibers. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Electrospinning is a unique and versatile approach for controlled one-dimensional microfabrication with fiber diameters typically ranging from nanometers to micrometers [1–4]. Due to its comparatively low cost, relatively high production rate, and simplicity of infrastructure and processing, electrospinning is being investigated extensively for various applications in materials processing and nanotechnology [5–9]. A typical electrospinning process involves the use of a high voltage bias to charge the surface of liquid droplets. When an applied electric field is sufficiently strong, charges built up on the surface of the droplets will overcome surface tension inducing the formation of a liquid jet that accelerates toward a grounded collector. As an electrospun jet approaches a collector, the jet experiences a fluid instability stage that leads to thinning of the jet and solidification of the fibers [10,11]. Because the effect of fluid instability limits the ability to weave the electrospun fibers, random-assembled membranes are generally obtained. Layer-by-layer electrospinning has been shown to produce fibrous porous membranes in several materials including metals, polymers, ceramics, and composites. Porous membranes have multiple applications and are actively used in energy storage, healthcare, biotechnology, and environmental engineering [5,7–9]. Femtosecond laser machining currently shows much promise as a versatile tool for the precise processing of materials with micro- and nano-scale features [12–14]. The unique character-
∗ Corresponding author at: Materials Science Program, Department of Mechanical Engineering, University of Rochester, 215 Hopeman Building, Rochester, NY 14627, USA. Tel.: +1 585 275 4844; fax: +1 585 256 2509. E-mail address: [email protected] (Y. Wu). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.09.111
istics of ultra-short laser pulses enable precise processing and minimize sample damage caused by stress waves, thermal conduction, and melting [15]. Researchers have utilized the femtosecond laser machining of ceramics, polymers, alloys, metals, semiconductors, and glass [16–22] to construct a wide range of devices in the biomedical, photonic, microelectronic, and microfluidic fields. In this paper, we demonstrate the ability of a high-intensity femtosecond laser to machine holes and slots in electrospun polycaprolactone (PCL) membranes. The laser system configuration allows for on-demand machining of arbitrary macro- and micropatterns into electrospun membranes, providing design flexibility that could be utilized to assess the structure, function, and application of electrospun membranes. 2. Experimental setup For fabrication of electrospun PCL membranes, a precursor solution of 10 (w/v%) was prepared by dissolving polycaprolactone (PCL, Aldrich, MW 80,000) in a solvent mixture of dichloromethane (Aldrich) and methanol (Aldrich) in a ratio of 8:2 by volume. The solution was loaded in a 10 ml plastic syringe (BD company) equipped with a 25 gauge nozzle (McMaster), and was pumped through the nozzle, using a syringe pump (Cole–Parmer), at a constant flow rate of 2.0 ml/h. A highvoltage power supply (Acopian) was used to provide a potential voltage of 18 kV to the stainless steel nozzle. Electrospun membranes were collected on grounded aluminum foil or carbon sheets. To create structures in electrospun PCL membranes, we used an amplified Ti:sapphire femtosecond laser system that consists of a mode-locked oscillator and a two-stage amplifier. The laser system generated 65-fs pulses containing 1.5 mJ per pulse at a
Y. Wu et al. / Applied Surface Science 257 (2011) 2432–2435
2433
Fig. 1. Experimental setup for machining of electrospun PCL membranes.
maximum repetition rate of 1 kHz and central wavelength of 800 nm. The experimental setup is shown in Fig. 1. The laser beam was horizontally polarized and focused by a thin lens onto a sample mounted on a motorized X–Y translation stage. The number of laser pulses incident on the sample was controlled by a fast electromechanical shutter. A beam splitter directed a fraction of the incident beam into a joulemeter so that the energy of the femtosecond pulses could be monitored. A neutral density filter was used to adjust the beam intensity incident on the sample plane. A desired pattern can be produced by opening the shutter and laterally translating the sample using the X-Y mechanized stage. The microstructure of the electrospun membranes before and after laser machining were characterized using a field-emission scanning electron microscope (FE-SEM, Zeiss-Leo DSM982 model) operated at an accelerating voltage of 5 kV. The samples were sputter coated with a thin film of gold prior to imaging.
Fig. 2. SEM micrograph of electrospun PCL membrane before femtosecond laser machining.
3. Results and discussion Fig. 2 shows a scanning electron microscope (SEM) image of a 100 m thick electrospun membrane. The diameters of the electrospun fibers range from 0.4 m to 0.9 m, with an average diameter of 0.5 m. The as-prepared membranes were observed to contain randomly distributed electrospun fibers. A series of experiments were conducted to assess the effect of laser fluence on structure fabrication in the electrospun PCL membranes. As shown in Fig. 3(a), using femtosecond laser pulses with a fluence
Fig. 3. SEM micrographs of electrospun membranes irradiated by femtosecond laser at different processing conditions: (a) F = 0.17 J/cm2 , (b) F = 0.75 J/cm2 , (c) F = 0.17 J/cm2 at a scanning speed of 1 mm/s, and (d) F = 0.75 J/cm2 at a scanning speed of 1 mm/s.
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Y. Wu et al. / Applied Surface Science 257 (2011) 2432–2435
Fig. 4. High magnification SEM micrographs of the regions circled in Fig. 3.
of 0.17 J/cm2 caused the electrospun fibers to melt together and form pores of varying size. The laser-irradiated zone has a very smooth surface with a spot size of about 360 m. The laser fluence was next increased to determine if continuous holes could be machined into the electrospun membrane without seriously disturbing the surrounding area. Fig. 3(b) shows a 600 m diameter hole that was produced using a laser fluence of 0.75 J/cm2 . Due to the Gaussian shape of the laser beam, a modified zone with an average width of 40 m can be seen around the machined hole. The scanning stage was next set to a speed of 1 mm/s to assess dynamic pattern transfer using the same laser fluences of 0.17 J/cm2 and 0.75 J/cm2 . The stage was programmed to create a rectangular pattern. Fig. 3(c) shows that the electrospun membrane has a smooth melting surface with an average width of 250 m, but that the laser energy is not sufficient to machine a hole through the electrospun fiber. As in the static case, pores are present in the irradiated area of the electrospun membrane indicating some degree of fiber melting. Increasing the laser fluence to 0.75 J/cm2 created a rectangular opening with a width of 500 m, as shown in Fig. 3(d). There is an affected zone with an average width of 60 m around the edge of the machined slot. The smooth morphology of the laser-machined edge suggests that the membrane was not significantly affected outside the laser irradiation area. The divisions between the electrospun fibers, the laserirradiated area, and the laser-affected zone can be clearly identified by examining the higher magnification SEM images in Fig. 4. These images also show the connections between electrospun and irradi-
ated fibers and allow for the identification of individual electrospun 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 distance between the centers of the circular holes is 1000 m, while the
Fig. 5. SEM image of an array of holes generated in electrospun membranes by femtosecond laser pulses at fluence of 0.6 J/cm2 .
Y. Wu et al. / Applied Surface Science 257 (2011) 2432–2435
diameter of the holes is approximately 436 m. The laser-machined holes are homogenously distributed within the electrospun membrane and the unexposed areas of the membrane seem to have retained their smooth morphology. Our study shows that femtosecond laser machining is an effective way to produce arbitrary structures in electrospun membranes. 4. Conclusion In summary, a high-intensity femtosecond laser was used to machine electrospun PCL membranes. Using femtosecond laser writing, circular holes and rectangular slots were precisely positioned and fabricated within the PCL membranes. The present study demonstrates that femtosecond laser machining is an efficient technique to prepare patterning structures in electrospun polymeric membranes. This technique could provide a useful platform for the fabrication of functional tissue scaffolds by allowing researchers to tailor the surface structure of scaffolds for specific biological responses. Acknowledgements Authors acknowledge Dr. Nathan Jenness for reading and editing the manuscript. This work was supported with NSF Grants (CMMI 0609265 and DGE 0221632) and AFOSR (FA9550-10-1-0067).
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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.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Femtosecond laser machining of electrospun membranes Yiquan Wu a,∗ , A.Y. Vorobyev b , Robert L. Clark a , Chunlei Guo b a b
Department of Mechanical Engineering, University of Rochester, Rochester, NY 14627, USA The Institute of Optics, University of Rochester, Rochester, NY 14627, USA
a r t i c l e
i n f o
Article history: Received 1 June 2010 Accepted 29 September 2010 Available online 27 October 2010 Keywords: Laser machining Electrospinning Membranes
a b s t r a c t We demonstrate that a femtosecond laser can be used to machine arbitrary patterns and pattern arrays into free-standing electrospun polycaprolactone (PCL) membranes. We also examine the influence of various laser irradiation settings on the final microstructure of electrospun membranes. A beam fluence of 0.6 J/cm2 is used to ablate holes in 100 m thick PCL membranes. The machined holes have an average diameter of 436 m and a center-to-center spacing of 1000 m. Based on these results, the femtosecond ablation of electrospun membranes shows great potential for fabricating a variety of functional tissue scaffolds. This technique will advance scaffold design by providing the ability to rapidly tailor surface morphology, while minimizing and controlling the deformation of the electrospun fibers. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Electrospinning is a unique and versatile approach for controlled one-dimensional microfabrication with fiber diameters typically ranging from nanometers to micrometers [1–4]. Due to its comparatively low cost, relatively high production rate, and simplicity of infrastructure and processing, electrospinning is being investigated extensively for various applications in materials processing and nanotechnology [5–9]. A typical electrospinning process involves the use of a high voltage bias to charge the surface of liquid droplets. When an applied electric field is sufficiently strong, charges built up on the surface of the droplets will overcome surface tension inducing the formation of a liquid jet that accelerates toward a grounded collector. As an electrospun jet approaches a collector, the jet experiences a fluid instability stage that leads to thinning of the jet and solidification of the fibers [10,11]. Because the effect of fluid instability limits the ability to weave the electrospun fibers, random-assembled membranes are generally obtained. Layer-by-layer electrospinning has been shown to produce fibrous porous membranes in several materials including metals, polymers, ceramics, and composites. Porous membranes have multiple applications and are actively used in energy storage, healthcare, biotechnology, and environmental engineering [5,7–9]. Femtosecond laser machining currently shows much promise as a versatile tool for the precise processing of materials with micro- and nano-scale features [12–14]. The unique character-
∗ Corresponding author at: Materials Science Program, Department of Mechanical Engineering, University of Rochester, 215 Hopeman Building, Rochester, NY 14627, USA. Tel.: +1 585 275 4844; fax: +1 585 256 2509. E-mail address: [email protected] (Y. Wu). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.09.111
istics of ultra-short laser pulses enable precise processing and minimize sample damage caused by stress waves, thermal conduction, and melting [15]. Researchers have utilized the femtosecond laser machining of ceramics, polymers, alloys, metals, semiconductors, and glass [16–22] to construct a wide range of devices in the biomedical, photonic, microelectronic, and microfluidic fields. In this paper, we demonstrate the ability of a high-intensity femtosecond laser to machine holes and slots in electrospun polycaprolactone (PCL) membranes. The laser system configuration allows for on-demand machining of arbitrary macro- and micropatterns into electrospun membranes, providing design flexibility that could be utilized to assess the structure, function, and application of electrospun membranes. 2. Experimental setup For fabrication of electrospun PCL membranes, a precursor solution of 10 (w/v%) was prepared by dissolving polycaprolactone (PCL, Aldrich, MW 80,000) in a solvent mixture of dichloromethane (Aldrich) and methanol (Aldrich) in a ratio of 8:2 by volume. The solution was loaded in a 10 ml plastic syringe (BD company) equipped with a 25 gauge nozzle (McMaster), and was pumped through the nozzle, using a syringe pump (Cole–Parmer), at a constant flow rate of 2.0 ml/h. A highvoltage power supply (Acopian) was used to provide a potential voltage of 18 kV to the stainless steel nozzle. Electrospun membranes were collected on grounded aluminum foil or carbon sheets. To create structures in electrospun PCL membranes, we used an amplified Ti:sapphire femtosecond laser system that consists of a mode-locked oscillator and a two-stage amplifier. The laser system generated 65-fs pulses containing 1.5 mJ per pulse at a
Y. Wu et al. / Applied Surface Science 257 (2011) 2432–2435
2433
Fig. 1. Experimental setup for machining of electrospun PCL membranes.
maximum repetition rate of 1 kHz and central wavelength of 800 nm. The experimental setup is shown in Fig. 1. The laser beam was horizontally polarized and focused by a thin lens onto a sample mounted on a motorized X–Y translation stage. The number of laser pulses incident on the sample was controlled by a fast electromechanical shutter. A beam splitter directed a fraction of the incident beam into a joulemeter so that the energy of the femtosecond pulses could be monitored. A neutral density filter was used to adjust the beam intensity incident on the sample plane. A desired pattern can be produced by opening the shutter and laterally translating the sample using the X-Y mechanized stage. The microstructure of the electrospun membranes before and after laser machining were characterized using a field-emission scanning electron microscope (FE-SEM, Zeiss-Leo DSM982 model) operated at an accelerating voltage of 5 kV. The samples were sputter coated with a thin film of gold prior to imaging.
Fig. 2. SEM micrograph of electrospun PCL membrane before femtosecond laser machining.
3. Results and discussion Fig. 2 shows a scanning electron microscope (SEM) image of a 100 m thick electrospun membrane. The diameters of the electrospun fibers range from 0.4 m to 0.9 m, with an average diameter of 0.5 m. The as-prepared membranes were observed to contain randomly distributed electrospun fibers. A series of experiments were conducted to assess the effect of laser fluence on structure fabrication in the electrospun PCL membranes. As shown in Fig. 3(a), using femtosecond laser pulses with a fluence
Fig. 3. SEM micrographs of electrospun membranes irradiated by femtosecond laser at different processing conditions: (a) F = 0.17 J/cm2 , (b) F = 0.75 J/cm2 , (c) F = 0.17 J/cm2 at a scanning speed of 1 mm/s, and (d) F = 0.75 J/cm2 at a scanning speed of 1 mm/s.
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Y. Wu et al. / Applied Surface Science 257 (2011) 2432–2435
Fig. 4. High magnification SEM micrographs of the regions circled in Fig. 3.
of 0.17 J/cm2 caused the electrospun fibers to melt together and form pores of varying size. The laser-irradiated zone has a very smooth surface with a spot size of about 360 m. The laser fluence was next increased to determine if continuous holes could be machined into the electrospun membrane without seriously disturbing the surrounding area. Fig. 3(b) shows a 600 m diameter hole that was produced using a laser fluence of 0.75 J/cm2 . Due to the Gaussian shape of the laser beam, a modified zone with an average width of 40 m can be seen around the machined hole. The scanning stage was next set to a speed of 1 mm/s to assess dynamic pattern transfer using the same laser fluences of 0.17 J/cm2 and 0.75 J/cm2 . The stage was programmed to create a rectangular pattern. Fig. 3(c) shows that the electrospun membrane has a smooth melting surface with an average width of 250 m, but that the laser energy is not sufficient to machine a hole through the electrospun fiber. As in the static case, pores are present in the irradiated area of the electrospun membrane indicating some degree of fiber melting. Increasing the laser fluence to 0.75 J/cm2 created a rectangular opening with a width of 500 m, as shown in Fig. 3(d). There is an affected zone with an average width of 60 m around the edge of the machined slot. The smooth morphology of the laser-machined edge suggests that the membrane was not significantly affected outside the laser irradiation area. The divisions between the electrospun fibers, the laserirradiated area, and the laser-affected zone can be clearly identified by examining the higher magnification SEM images in Fig. 4. These images also show the connections between electrospun and irradi-
ated fibers and allow for the identification of individual electrospun 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 distance between the centers of the circular holes is 1000 m, while the
Fig. 5. SEM image of an array of holes generated in electrospun membranes by femtosecond laser pulses at fluence of 0.6 J/cm2 .
Y. Wu et al. / Applied Surface Science 257 (2011) 2432–2435
diameter of the holes is approximately 436 m. The laser-machined holes are homogenously distributed within the electrospun membrane and the unexposed areas of the membrane seem to have retained their smooth morphology. Our study shows that femtosecond laser machining is an effective way to produce arbitrary structures in electrospun membranes. 4. Conclusion In summary, a high-intensity femtosecond laser was used to machine electrospun PCL membranes. Using femtosecond laser writing, circular holes and rectangular slots were precisely positioned and fabricated within the PCL membranes. The present study demonstrates that femtosecond laser machining is an efficient technique to prepare patterning structures in electrospun polymeric membranes. This technique could provide a useful platform for the fabrication of functional tissue scaffolds by allowing researchers to tailor the surface structure of scaffolds for specific biological responses. Acknowledgements Authors acknowledge Dr. Nathan Jenness for reading and editing the manuscript. This work was supported with NSF Grants (CMMI 0609265 and DGE 0221632) and AFOSR (FA9550-10-1-0067).
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.
