# Femtosecond Laser Processing of Biodegradable Polymers
## 3. Degradable Property after Laser Irradiation
Laser fluences and number of pulses were carefully selected to fabricate laser ablation craters of the same diameter to minimize the initial difference in the diameter. A crater fabricated by mechanical milling is also shown for comparison. The crater fabricated by femtosecond laser pulses at 800 nm in wavelength showed no significant change in diameter after 48 h of immersion in phosphate-buffered saline (PBS), whereas that by 400-nm femtosecond laser pulses exhibited comparably rapid expansion over an immersion period, indicating the acceleration of degradation. The acceleration of degradation can also be confirmed by the mass change. As shown in Figure 4, there was a significant decrease in the mass of the samples irradiated by 400 nm laser pulses. These results demonstrate the potential of femtosecond laser processing for controlling the degradation rate after form shaping and structuring by selecting a suitable wavelength. The degradation rate of a biodegradable polymer after laser irradiation has been explained by the decrease in crystallinity associated with surface melting, and in molecular weight owing to the photolysis of chemical bonds. Because PLGA is a polymer with less crystallinity, with the wavelength of 400 nm, the significant acceleration of the biodegradation in the case of PLGA [40] is attributable to the decrease in molecular weight induced by the chemical bond dissociation rather than the change in crystallinity. Figure 3: Digital microscopic images of craters formed on PLGA films. The time shown in each figure indicates the time of the samples immersed in PBS at 37 \({}^{\circ}\)C. (**a**) Crater formed by mechanical milling. (**b**) Laser ablation crater under the condition of 800 nm wavelength, 1.0 J/cm\({}^{2}\), 15,000 pulses. (**c**) Laser ablation crater under the condition of 400-nm wavelength, 0.15 J/cm\({}^{2}\), 15,000 pulses. Laser fluences were carefully selected to fabricate laser ablation craters of the same diameter to minimize the initial difference in the diameter. Laser-induced periodic surface structure (LIPSS) is a unique nanostructure formed by laser irradiation of multiple pulses at laser fluence under or near the threshold of single-pulse laser ablation. Because an LIPSS can be formed in the laser-irradiated area with unfocused or loosely focused laser pulses, a large area can be treated, i.e., the formation throughput of subwavelength periodic structures is comparably high. Many studies on LIPSS formation on metals, dielectrics, and semiconductors have been reported. The formation of LIPSS on titanium [57] and titanium oxide [58] has been studied for enhancing the adhesion of cells and alignment toward biomedical applications. Studies on LIPSS formation on the surface of polymers can be traced back to a publication in 1985 [59]; however, investigations with biodegradable polymers are limited to a few cases. [PERSON] et al. demonstrated the formation of LIPSS on chitosan and polyvinylpyrrolidone, highly water-soluble biodegradable polymers [60]. They used nanosecond laser pulses at wavelengths of 193, 213, and 266 nm; therefore, linear absorption was the dominant interaction. Periodic structures formed on a PLLA surface by using femtosecond laser pulses are shown in Figure 5. Scanning electron microscopy (SEM) images are shown for the cases of 800, 400, and 266 nm for comparison. At 800 nm, LIPSS was rarely formed only under limited experimental conditions (Figure 5a). The structure perpendicular to the laser polarization had a 149-nm average periodicity, much shorter than the laser wavelength. Figure 5b shows LIPSS formed with a 400-nm laser wavelength. Compared to the case at 800 nm, LIPSS perpendicular to the polarization was formed at wide laser fluence and pulse duration conditions. Because the photon energy at 400 nm is higher than at 800 nm, the number of photons necessary for multiphoton absorption is smaller at 400 nm, resulting in the facilitation of Coulomb explosion or bond dissociation mediated by Norrish type II reaction.