# Femtosecond Laser Processing of Biodegradable Polymers
## 3. Degradable Property after Laser Irradiation  
The degradable property of biodegradable polymers affects the durability and biocompatibility of the fabricated structure in addition to the sustainability of the structure in practical applications. Controlling the degradation property is one of the keys to effective utilization, which may depend on the applications and patients. The biodegradation of a biodegradable polymer in a physiological environment proceeds via the hydrolysis of ester bonds in the presence of absorbed water. The degradation rate of a biodegradable polymer can be influenced by several factors including molecular weight, crystallinity, surface morphology, and surface hydrophilicity. Several methods have been proposed to control these factors, such as annealing [49], copolymerization [50], porous structure formation [51], and light-based modification [52; 53; 54; 55]. Laser-based methods enable high spatial selective modification, which provides localized changes in the biodegradable property, even after molding. Because the change in biodegradable properties by laser irradiation is an emerging technology, papers describing the direct evaluation on biodegradable properties are limited, although the crystallinity, molecular weight, and bond dissociation after laser irradiation have been studied widely. In 2005, a study on the change in biodegradation rate by laser processing was reported. [PERSON] et al. studied the laser ablation of polycaprolactone (PCL) and polyglycolic acid (PGA) by using ArF excimer laser (wavelength 193 nm) [12]. They reported the scission of chemical bonds and the generation of CO and CO\({}_{2}\) gases, which was analyzed by X-ray photoelectron spectroscopy (XPS). Laser-irradiated PCL exhibited a high degradation rate compared to non-irradiated PCL, whereas PGA showed no significant change in degradation rate. Controlling the degradation rate was also attempted by taking advantage of the thermal effect of laser pulse because the degradation rate of the biodegradable polymers depends largely on the molecular structure. Laser pulses comparable to or longer than nanoseconds have been used for obtaining thermal effects, i.e., laser annealing. By using a laser, the annealing depth can be controlled by changing the wavelength. Because of the different penetration depths in a material, there is high spatial controllability of both the depth and the irradiated area, since the laser irradiation area is easily controlled by the beam diameter and laser scanning. [PERSON] et al. investigated the decrease in the crystallinity of PLLA after laser pulse irradiation (wavelength 248 nm, pulse duration 25 ns), which was evaluated by wide-angle X-ray diffraction (WAXD) [54]. Figure 2. XRD spectra for the surface of PLLA films. Black, red, and purple lines indicate non-irradiated films (control), films irradiated with a nanosecond laser, and films irradiated with a femtosecond laser, respectively. A wavelength of 266 nm was used for both cases femtosecond laser pulses and nanosecond laser pulses. On the basis of the decrease in the ratio of O1s to C1s peaks, they discussed the dissociation of C-O bonds and the possible emission of oxygen atoms from the film in the form of CO and CO\({}_{2}\). [PERSON] et al. reported a decrease in the wettability of PLLA with laser fluence and number of pulses; the latter has a more significant influence [56]. Femtosecond laser processing of biodegradable polymers has advantages in 3D precise processing; however, detailed studies on the effects of femtosecond laser irradiation on the degradable property are limited to a few cases. Our group found different degradation rates of the biodegradable polymer after femtosecond laser ablation, which depends on the laser wavelength [40]. Figure 3 shows a comparison of laser ablation craters on a PLGA surface at different times after irradiation with 800-nm and 400-nm femtosecond laser pulses.