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Laser-induced periodic surface structure (LIPSS) is one of the most remarkable nanostructures formed only by a simple procedure of laser irradiation that enables to control cell behaviors. To the best of our knowledge, however, LIPSS formation on a scaffold-usable biodegradable polymer had not been succeede d probably due to relatively-low glass transition temperature and melting temperature of such polymers. In this study, we demonstrate LIPSS formation on a poly-L-lactic acid (PLLA), a versatile biodegradable polymer which has been widely used in clinical practice. Experimental results revealed that the repetition rate of femtosecond laser is one of the key parameters for LIPSS formation on PLLA, suggesting that thermal properties and photochemical reactions should be considered. The present study expands the potential of femtosecond laser processing for fabrication of highly-biocompatible scaffold in tissue engineering.
© 2015 Optical Society of America
1. Introduction
Biodegradable polymers have been receiving increasing attention in wide field of applications including optical devices [1] as well as a scaffold in tissue engineering not only because of their controllability of degradation but also high mechanical strength [2]. Several methods for manufacturing or processing biodegradable polymers including soft lithography [3] and photolithography [4] have been studied for medical applications. Among these methods, laser processing has advantages in dry processing without using a toxic chemical component and in ease of processing complex-shaped surfaces after molding. As most of biodegradable polymers are transparent in visible wavelengths, they give an absorption band in the ultraviolet range and thus short-wavelength lasers have been used to process biodegradable polymers [5]. In the last decade, ultrashort pulsed laser realized precise processing of biodegradable polymers via multiphoton absorption with minimized heat affected zone (HAZ) [6]. Wang et al. implemented femtosecond laser processing for fabrication of pillared microvessel scaffolds of polylactic-co-glycolic acid (PLGA) consisting of tens of micrometers pillared branches [7]. Direct laser writing in micrometer scale is attractive to be used in surface structuring for cell alignment without affecting the cell adhesion [8,9] as well as in fabrication of 3D scaffolds via multi-photon polymerization for neural tissue engineering [10]. It was also reported that microstructures fabricated by femtosecond laser promotes up-regulation of myogenic markers [11]. Ultrashort pulsed laser microprocessing, that is to say, is becoming a promising candidate for fabrication of implantable devices made of biodegradable polymers.
Since most of biodegradable polymers have hydrophobic properties, enhancement of cell adhesiveness is one of the key factors for successful scaffold fabrication in tissue engineering. Tiaw et al. reported that surface modification of poly(ε-caprolactone) (PCL) using femtosecond laser as well as nanosecond laser enhances the wettability of the hydrophobic nature of the PCL membrane [12]. Lee et al. demonstrated that femtosecond laser ablation increases cell infiltration into nanofibrous scaffolds [13]. To enhance cell adhesiveness, surface nanostructures are more likely to play an important role instead of microstructures, i. e. the microprocessing feature of the femtosecond laser processing provides a method to fabricate complex frameworks while the nanoprocessing feature has potential to control biocompatibility on the surface.
Laser-induced periodic surface structure (LIPSS) is one of the most remarkable nanostructures formed only by a simple procedure of laser irradiation of multiple pulses at laser fluence under or near the single pulse ablation threshold. Unlike laser processing with tightly-focused laser beam, a large area is able to be treated because subwavelength periodic structures are obtainable even in unfocused laser irradiated area. LIPSS was first observed in 1965 [14], and the development of ultrashort pulsed laser in the 1990s opened a new door for highly-precise and controlled LIPSS including high spatial frequency LIPSS (HSFL). LIPSS formation on various materials such as metals [15,16], semiconductors [17,18], and dielectrics [19–21] has been investigated in detail. It is notable that LIPSS formed on titanium [22] and titanium oxide [23] have been reported to show effect on cell behaviors such as cell adhesion, spreading, and orientation.
A few studies of LIPSS formation on polymers by femtosecond laser irradiation have also been reported [24,25]. Rebollar et al. reported a detailed study that low spatial frequency LIPSS (LSFL) was formed parallel to the laser polarization at the periphery of ablation crater on the surface of polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), and polycarbonate (PC) with femtosecond laser pulses [26]. To the best of our knowledge, however, successful LIPSS formation on biodegradable polymers has not been reported except in the case of poly(vinyl pyrrolidone), a water-soluble polymer, by using ultraviolet nanosecond laser pulse [27]. This may be due to relatively-low grass-transition temperatures and melting temperatures of scaffold-usable biodegradable polymers. If LIPSS is successfully formed on the scaffold-usable biodegradable polymers, advanced scaffolds would be realized by a single system owing to both the microprocessing and nanoprocessing features of femtosecond laser.
The aim of this study is to demonstrate LIPSS formation on a biodegradable polymer in which the material is usable for a scaffold in tissue engineering. LIPSS formation on a surface of poly-L-lactic acid (PLLA), a versatile biodegradable polymer which has widely been used in clinical practice, was investigated by irradiating multiple pulses of femtosecond laser. We first investigated the ablation threshold of PLLA since LIPSS is formed at laser fluence under or near the single pulse ablation threshold. Then, we analyzed the chemical structure on the surface of PLLA by X-ray photoelectron spectroscopy (XPS) before and after laser irradiation. Based on the results, we revealed critical conditions of laser irradiation for LIPSS formation on PLLA.
2. Experimental
Poly-L-lactic acid film was purchased from BMG Inc. (Kyoto, Japan). Weight-average molecular weight, degree of crystallinity, glass transition temperature, and melting temperature are 191781, 51.94%, 56.400C, and 180.010C, respectively. The thickness of the film was 0.5 mm.
Linearly polarized femtosecond laser pulses from a Ti:sapphire chirped pulse amplification laser system (Libra, Coherent, Inc., Santa Clara, California) operating at a central wavelength of 800 nm (fundamental wave) or 400 nm (second harmonic wave) were focused onto a surface of PLLA film at normal incidence using a plano-convex lens with a focal length of 200 mm. The laser was operated in external trigger mode, which provides desired number of pulses at a repetition rate of 1 kHz or 100 Hz. Pulse duration before the lens was ~100 fs at 800 nm central wavelength measured with an autocorrelator.
The surface of the samples after laser irradiation was observed by scanning electron microscopy (SEM). Ablation threshold of PLLA was determined from the laser fluence and the ablated area. Periodicities of LIPSS formed on a PLLA surface were determined from the Fourier transform of SEM images. To evaluate the dispersion of the periodicity, respective periods were also measured for more than 10 grooves.
Modification of chemical structure of the PLLA surface before and after laser irradiation was analyzed by XPS. Samples for the XPS analysis were prepared by scanning focused femtosecond laser pulses (repetition rate of 1 kHz, spot size of 330 μm) on surfaces of PLLA at normal incidence using the plano-convex lens. The laser fluences were 1.0 J/cm2 and 0.30 J/cm2 for 800 nm and 400 nm, respectively, and the scanning speed was fixed to 66 μm/s.
3. Results
3.1. Ablation threshold laser fluence of PLLA film
Dependences of ablated area of PLLA surface on laser fluence with a single pulse and that with 300 pulses at 800 nm are shown in Figs. 1(a) and 1(b), respectively. From the figures, laser ablation thresholds with a single pulse and that with 300 pulses were extrapolated to 1.6 J/cm2 and 0.45 J/cm2, respectively. The ablation threshold with 300 pulses is lower than that with a single pulse. This difference is attributable to the incubation effect [28]. We also conducted a similar experiment with the second harmonic wave, 400 nm. Since the breakdown in air occurred at laser fluence below the single pulse ablation threshold before the laser pulse arrived at the surface of PLLA, single pulse ablation threshold at 400 nm was not able to be determined. The laser ablation threshold at 400 nm with 300 pulses was extrapolated to 0.27 J/cm2.
Fig. 1 Dependences of ablated area on laser fluences for extrapolating ablation thresholds. The ablated area was determined by the average of 5 samples under irradiation conditions. (a) A single pulse at 800 nm, (b) 300 pulses at 800 nm, and (c) 300 pulses at 400 nm. Error bars indicate standard deviation.
3.2. XPS analysis of PLLA film before and after laser irradiation
Wide XPS spectra of PLLA surface before and after femtosecond laser irradiation are shown in Fig. 2(a). No significant change in the peaks of binding energies can be observed. C1s spectra and O1s spectra are shown in Figs. 2(b) and 2(c), respectively. As shown in Fig. 2(b), the C1s spectra consists of three peaks; carbon in C-H bond at 285 eV, neighboring carbon in C-O bond at 287 eV and carbon in carboxylic group (C = OO) at 289 eV. Decrease of the peak at 285 eV and increase of the other peaks are observed. The O1s spectra is expected to be composed of two peaks; oxygen in O = C bond at 532 eV and O-C bond at 534 eV. In Fig. 2(c), however, those two peaks overlapped and only one gradual peak is observed as similar to the XPS spectra shown in [29,30].
Fig. 2 XPS analysis results of PLLA surface before and after laser irradiation. (a) Wide XPS spectra, (b) C1s spectra, and (c) O1s spectra. Black lines, red lines, and blue lines indicate PLLA surface before laser irradiation, PLLA surface after irradiation at 800 nm at 1.0 J/cm2, and PLLA surface after irradiation at 400 nm at 0.30 J/cm2, respectively. The scanning speed was 66 μm/s for (b) and (c).
3.3. Comparison of femtosecond-LIPSS formation by 800 nm and 400 nm in wavelengths
We attempted to demonstrate LIPSS formation on PLLA by using femtosecond laser of 800 nm or 400 nm. Femtosecond laser pulses at 800 nm with laser fluences varying from 0.10 to 1.0 J/cm2 and number of pulses varying from 10 to 15000 were irradiated on PLLA surface. As a result, LIPSS was formed only under the irradiation condition of 1.0 J/cm2 with 10000 pulses. The HSFL was formed perpendicular to the laser polarization with an average periodicity of 149 nm as shown in Fig. 3(a). Femtosecond laser pulses at 400 nm in wavelength with laser fluences varying from 0.01 to 1.0 J/cm2 and number of pulses varying from 100 to 15000 were irradiated on PLLA. Compared to the results with 800 nm, LIPSS was formed under a certain wide range of irradiation conditions. The SEM images of LIPSS formed with 400 nm at the laser fluence of 0.30 J/cm2 for 3000 pulses is shown in Fig. 3(b). LIPSS was observed at peripheries of ablation craters in which HSFL was formed perpendicular to the laser polarization with an average periodicity of 101 nm. Figure 4 shows average periodicities of successfully formed LIPSS under different number of pulses (a) and laser fluences (b). Average periodicity was obtained from fast Fourier transform (FFT) for five SEM images of LIPSS for each condition. With 800 nm, LIPSS was formed under a limited laser irradiation condition of laser fluence 1.0 J/cm2 for 10000 pulses, which is shown as a single open circle in Fig. 4(a). With 400 nm, LIPSS has almost identical average periodicity in the range of 3000 to 15000 laser pulses as shown in Fig. 4(a). In addition, no significant change in average periodicity was observed for different laser fluences as shown in Fig. 4(b). Although the underlying physics of HSFL formation is still an open question, our results with PLLA show no significant change in periodicity of HSFL except the result for different wavelength, which is similar to the cases of metals and dielectrics. LSFL was neither observed under the irradiation of 800 nm nor 400 nm.
Fig. 3 SEM images of LIPSS formed on PLLA: (a) Laser irradiated surface at 800 nm, 1.0 J/cm2, 10000 pulses and (b) that at 400 nm, 0.20 J/cm2, 5000 pulses. Average periodicities of LIPSS (HSFL) were measured to be 149 nm (a) and 101 nm (b). Scale bars represent 5 μm.
Fig. 4 Dependences of average periodicity of LIPSS formed on PLLA on (a) number of laser pulses and on (b) laser fluence. Values show an average of five measurements ± standard deviation.
3.4. Repetition rate dependence on LIPSS formation
Based on the results shown in section 3.3, we employed 400 nm in irradiation wavelength for more detailed study on LIPSS formation. Figure 5 shows SEM images of typical surface structures after femtosecond laser irradiation to PLLA film at different repetition rates of 100 Hz and 1 kHz. Different structures such as ablation [Figs. 5(b) and 5(e)], LIPSS formation [Figs. 5(c) and 5(f)], and modification [Fig. 5(d)] were observed by changing laser irradiation conditions of repetition rate, laser fluence, and number of pulses. If LIPSS formation was observed, the result was categorized to the LIPSS formation. Significant removal induced by laser ablation without LIPSS formation was categorized to ablation. Change in roughness by laser irradiation without LIPSS formation or significant removal was categorized to modification. At the repetition rate of 100 Hz, large LIPSS formation area was observed with less ablation area under the condition of 0.20 J/cm2 for 5000 pulses while significant removal by laser ablation was observed at 1 kHz. The LIPSS formation area was small and scattered at the periphery of ablation crater at 1 kHz.
Fig. 5 SEM images of typical PLLA surface before and after laser irradiation at 400 nm: (a) Before laser irradiation, (b, c) after laser irradiation at repetition rate of 100 Hz, and (d,e,f) after laser irradiation at repetition rate of 1 kHz. (b, e) Laser ablation without LIPSS formation. (c, f) LIPSS was observed in the irradiated area. (d) Modification without significant laser ablation and LIPSS formation. Laser irradiation conditions were 0.25 J/cm2 for 3000 pulses (b), 0.20 J/cm2 for 5000 pulses (c), 0.10 J/cm2 for 5000 pulses (d), 0.30 J/cm2 for 1000 pulses (e), and 0.20 J/cm2 for 5000 pulses (f). Scale bars represent 10 μm.
Figure 6 summarizes surface conditions after laser irradiation at 400 nm at the repetition rate of 100 Hz [Fig. 6(a)] and that of 1 kHz [Fig. 6(b)], which were categorized to ablation, LIPSS formation, modification, and no modification. LIPSS was formed with more than 5000 laser pulses at laser fluence higher than 0.10 J/cm2 at 100 Hz. At 1 kHz, LIPSS was more likely to be observed in relatively wide conditions; however, at the range of laser fluence from 0.075 to 0.15 J/cm2, randomly formed small LIPSS were observed without ablation craters (data not shown). It should be noted in summary that LIPSS is able to be formed under a wider range of irradiation conditions at 1 kHz than that at100 Hz, while a large area of LIPSS formation with a smoother surface is obtainable with 100 Hz. Figure 7 shows enlarged images of LIPSS formed at 100 Hz with different number of pulses. HSFL was formed perpendicular to the laser polarization with an average periodicity of 104 and 101 nm for 5000 and 12000 pulses, respectively. The standard deviations were 42 nm for 5000 pulses while 28 nm for 12000 pulses. Although the periodicities were comparable, increase in the number of pulses decreased the dispersion of the periodicity, which resulted in the formation of well-periodical ripple structure.
Fig. 6 Surface conditions after laser irradiation at 400 nm with varying number of pulses and laser fluence. (a) Repetition rate of 100 Hz. (b) Repetition rate of 1 kHz. Results are categorized to ablation(♦), LIPSS formation(●), modification(∆) and no modification(x).
Fig. 7 Large magnification SEM images of PLLA surface after laser irradiation at 100 Hz, 0.20 J/cm2 with different numbers of pulses: (a) 5000 pulses and (b) 12000 pulses. Scale bars represent 1 μm.
4. Discussion
No remarkable change of peaks in wide XPS spectra can be observed [Fig. 2(a)]. This result suggests no significant structural change occurred after irradiation and thus the basic properties of PLLA such as biodegradability may be considered to remain unaltered. Changes in the XPS spectra indicate the changes in the ratios of chemical bond in the surface of PLLA. Decrease of the peak at 285 eV in Fig. 2(b) and increase of the other peaks in Figs. 2(b) and 2(c) indicate C-H bond breakage and increase of C-O and C = O bonds. Chemical bond energies of each bond are 3.6 eV for C-C bond, 3.8 eV for C-O bond, 4.3 eV for C-H bond and 7.5 eV for C = O bond. Since photon energies of 800 nm and 400 nm are 1.55 eV and 3.10 eV, respectively, multiphoton absorption should be required for photolysis of chemical bonds. Results shown in the figure are attributed to the increase in temperature over the glass transition temperature in some degree.
The lower ablation threshold with 300 pulses at 400 nm compared to that at 800 nm as shown in Fig. 1 indicates that laser pulses at 400 nm are more likely absorbed via multiphoton absorption by PLLA due to the larger photon energy at shorter wavelength. The difference in LIPSS formation at 800 nm and that at 400 nm is also able to be explained by a difference in photon energies. The absorption edge wavelength of PLLA is approximately 225 nm [30] and thus multiphoton absorption is required to excite electrons over the bandgap. Electron excitation from valence band to conduction band by two-photon absorption is likely to be occurred at 400 nm compared to four-photon absorption at 800 nm.
It was reported that LIPSS is formed at superficial temperature over the glass transition temperature and under the melting temperature in the case of polymers [26]. The difference of LIPSS formation depending on repetition rate [Fig. 6] might be explained with the difference in superficial temperature induced by heat accumulation. The lower superficial viscosity caused by increase of superficial temperature is considered to be an important factor for LIPSS formation on polymers by laser irradiation with pulse duration longer than nanoseconds [32]. In femtosecond laser processing, it is widely known that the heat accumulation is negligible at pulse energy lower than 900 nJ at repetition rate less than 200 kHz in the case of borosilicate glass [33]. Although the repetition rate used in our study is quite less compared to 200 kHz, the pulse energy applied for LIPSS formation was over 15.4 μJ and the physical constants of the material are much different. The melting temperature, thermal diffusivity and heat capacity of PLLA are 180°C, 0.125 mm2/s and 1.0J/g•K [34], respectively. Compared to the constants of borosilicate glass, PLLA need more energy to raise the temperature because of its larger heat capacity. Due to its smaller thermal diffusivity and lower melting temperature, it is easier for PLLA to accumulate the heat and reach the melting temperature.
The difference in the LIPSS formation on the repetition rate may also attributable to shielding by the ablation plume as well as low-density plasma. Future experiments in vacuum chamber would reveal the effect of ablation plume. In the case of biological tissue, low-density plasma by multiphoton absorption and ionization was reported [35]. Laser ablation at pulse energies below the optical breakdown threshold is largely depends on the photochemical reactions. In [35], K. Kuetemeyer et al. reported that the increase of the laser-induced damage was observed with decreasing repletion rate, whose behavior is opposite to the case of borosilicate glass. The chemical reactions are disturbed by following pulse at the time intervals less than 1 ms because the reactions continue for approximately 10−3 s [36]. In our study, LIPSS was formed under a wider range of irradiation conditions at 1 kHz than that at 100 Hz, however, large area of LIPSS formation on a smoother surface is obtainable with 100 Hz compared to 1 kHz. Although further study is needed, it should be noted that presented results show photochemical reactions depending on the repetition rate as well as the thermal properties are the key factors for LIPSS formation on a biodegradable polymer.
5. Conclusion
In this study, we demonstrated the formation of ripple nanostructure on PLLA by using femtosecond laser. LIPSS (HSFL) was formed perpendicular to the laser polarization. The repetition rate of femtosecond laser is one of the key parameters for LIPSS formation on a biodegradable polymer, suggesting that thermal properties and photochemical reactions should be considered. To the best of our knowledge, this is the first report of LIPSS formation on a scaffold-usable biodegradable polymer by using femtosecond laser. The presented study can open up wide potential of femtosecond laser processing for fabrication of high-biocompatible scaffold in tissue engineering.
Acknowledgment
This works was supported in part by KAKENHI Grant-in-Aid for Young Scientists (A) No. 26702019.
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