Objective In recent years, significant progress has been made in smart actuation devices; they have great potential for applications in microsurgery, cell manipulation, and biosensing. The majority of conventional actuators are mechatronic devices made of hard materials (e.g., metals, silicon, and silica) with low biocompatibility, softness, flexibility, and biodegradability, limiting their use in biomedical domains. Therefore, smart soft actuators made of soft and adaptive materials have significant advantages in this area. Biocompatible materials that can be directly applied to biomedical applications, such as cell manipulation and tissue sectioning, have significant advantages for future smart soft actuators. Thus, studying the treatment of biocompatible materials to create smart soft actuators critical for the development of intelligent micro-robots and micromechanical systems is crucial. In this paper, three-dimensional (3D) micromechanical structures based on biocompatible BSA (bovine serum albumin) material are directly written using the femtosecond laser two-photon polymerization technique. The femtosecond laser’s high 3D processing capabilities allow for 3D direct writing of the micro-nano devices’ external contours while modulating the properties of the materials inside the structure. The relationship between the degree of micromechanical expansion under external environmental stimulation and the bending angle, as well as the size of the mesh density, is examined using the design technique presented in this research. Furthermore, a 3D robotic arm model with capture and release capability is designed and fabricated. In the future, this design approach will has numerous applications in biomedical engineering, optical systems, micromechanical systems, and other domains.
Methods In this study, BSA serves as the monomer, and MB (methylene blue) serves as the photoinitiator for the experiments. To make a viscous protein hydrogel, 500 mg/mL BSA and 0.6 mg/mL MB were dissolved in ultrapure water (18.2 MΩ·cm, 25 ℃). The prepared gels were then incubated at 4 ℃ for 24 h in a dark box to completely dissolve the constituent components. A homemade femtosecond laser two-photon polymerization processing equipment was used to obtain the BSA microstructures. For the fabrication, 20 μL BSA hydrogel was charged into a glass substrate, and a PDMS (polydimethylsiloxane) cavity was used to limit the evaporation of water in the gel. A Ti:sapphire femtosecond laser oscillator (Spectra-Physics 3960-X1BB) with a central wavelength of 800 nm, a pulse width of 120 fs, and an 80 MHz repetition rate generate the laser beam. A 60× oil immersion objective is employed to concentrate the femtosecond laser closely into the hydrogel for point-by-point 3D scanning. Before employing the objective, 20 mW-average laser power was measured, and the exposure duration for each voxel was 1000 μs. The samples were thoroughly rinsed in ultrapure water for few minutes after fabrication to remove the unpolymerized hydrogel.
Results and Discussions The swelling ratio of BSA micro-nano blocks increased as the scanning step length increased from 100 nm to 200 nm because of a decrease in the cross-linking density of the processed structures. In a pH 1 solution, the swelling ratio is about 1.9 when the step length is 200 nm, whereas in a pH 13 solution, the swelling ratio is 2.65. For a step length of 100 nm in an alkaline solution, the swelling ratio is 76.7% of a 200 nm step length fabricated structure. In this paper, femtosecond laser two-photon polymerization was used to fabricate three bilayer cantilever structures with different scan step length distributions of 50 nm/100 nm,50 nm/150 nm,and 50 nm/200 nm. According to the experimental results, the maximum rotation angle of the bilayer cantilever structure is 28° when the bilayer scanning step length is 50 nm/200 nm. As shown in Figure 5(b), the micro-machine was subsequently employed to capture a micro-nano rod structure with a width of 6 μm. When switched to the solution with pH 13, the micromechanical “arm” swells and “grip” the rod tightly. The “release” operation can be completed quickly by switching back to the pH 5 solution. The experiment can be repeated over and over again.
Conclusions This work describes a method for fabricating micromechanical devices using pH-responsive materials with nonuniform internal lattice density. Here, 3D micro-nano devices with high precision can be flexibly designed and produced using a femtosecond laser two-photon polymerization technique. It is also demonstrated that soft smart micro-machine with selective and accurate trapping and releasing may be obtained utilizing this technique by investigating the pH-responsive characteristics of BSA biomaterials. The construction of smart micro-nano devices with extensive and unprecedented functionality can be easily accomplished due to the rapid development of novel fabrication techniques and functional materials. Such smart micro-machines produced from biocompatible protein materials will play an essential role in future applications such as biomedical detection, microcellular analysis, and bionanotechnology because of advanced laser integration technology.
