# Femtosecond Laser Processing of Biodegradable Polymers
## 1 Introduction  
Cells, growth factors, and scaffolds are essential elements for the regeneration of a three-dimensional (3D) structure in tissue engineering. Tissue scaffolds play a role as a matrix for cell proliferation until the tissue is regenerated. Consequently, biodegradable polymers are considered promising materials for tissue scaffolding because of their degradable properties. Repeated surgery for the removal of the scaffold is not necessary after tissue regeneration if the degradation of the scaffold in a body can be controlled for an appropriately sustained period of time. The biocompatibility of biodegradable polymers is comparably high, which realizes the translation of the materials into practical medical applications such as bone fixation material and sutures. To maximize the utility and potential of biodegradable polymers, technologies to fabricate the material into an arbitrary shape, i.e., processing technologies, are crucial. In particular, customization in shape forming is required in many cases in biomedical applications to fabricate tailored products for patients. Laser processing enables such tailored fabrication because of its suitability for computer-aided scanning and processing without a mold, which shows great potential for use in the technological transition from mass production to mass customization.  
Laser processing of biodegradable polymers has many advantages in addition to its simple processing. A chemical solvent, whose residue may be toxic for the human body, is not necessary for laser-based fabrication. Additional microstructuring in the localized area is easy to perform even after the formation of a general shape. Because many biodegradable polymers show large optical absorption in ultraviolet (UV) wavelengths and less absorption at visible to near-infrared wavelengths, lasers oscillating at UV wavelengths have been used for processing in cases of nanosecond laser-based fabrication [1; 2; 3; 4]. [PERSON] et al. in 2001 reported the laser ablation of polyvinyl alcohol by using an XeCl excimer laser (308 nm in wavelength) and fourth harmonics generation (FHG) ofa Nd:YAG laser (266 nm in wavelength) [1]. They reported that the heat-affected zone (HAZ) was relatively smaller while using UV wavelengths. [PERSON] et al. formed porous structures composed of micrometer-sized bubbles on the surface of collagen, chitosan, and polyvinylpyrrolidone (PVP) by using a KrF excimer laser (248 nm) [4]. However, because femtosecond lasers enable the processing of transparent materials via nonlinear optical interaction, femtosecond lasers have opened up new avenues to fabricate three-dimensional (3D) structures with visible and near-infrared wavelengths.  
The degradation rate of biodegradable polymers depends on the crystallinity and composed micro- and nanostructures. Biodegradability can be changed by laser irradiation because laser pulses induce the scission of chemical bonding. In addition, laser processing enables the formation of not only entire shapes but also micro- and nano-sized substructures. Recently, considerable attention has been given to laser-based surface structuring of biopolymers for tissue engineering and for drug delivery systems because of these advantages.  
In this review, the fundamentals and applications of femtosecond laser processing of biodegradable polymers are described by reviewing previous studies. A significant increase in the number of papers on femtosecond laser processing of biodegradable polymers has been seen in the last decade. Laser processing properties are introduced, along with micro- and nanostructuring, changes in biodegradability by laser irradiation, and experimental demonstrations for potential applications.