Due to population aging and change in the modern lifestyle, tens of thousands of people are troubled by orthopedic, oral, and facial diseases. The demand for high-quality medical devices and implants in clinical medicine is increasing. Compared with traditional inorganic nonmetallic and polymer materials, metal materials have better biomechanical properties and processing ability. The surface state of medical devices and implants is an important factor in therapeutic schedules since it affects the complex biological behavior of nearby tissues, such as cell proliferation and differentiation, bone integration, immune response, neurotransmitter release and transport, bacterial infection, and so on. To promote innovation and development in the medical field, it is imperative to develop a simple, efficient, practical, and reliable preparation method for high-performance biological functional surfaces of typical medical devices and clinical implants.
Recently, many methods for modifying surfaces of medical metal materials have been developed. Various research methods focus on regulating the corrosion resistance and degradation rate of implants, blocking the release of harmful elements, promoting the adaptation of mechanical properties between implants and biological tissues, increasing biocompatibility, and obtaining antibacterial surfaces. Common surface modification methods include anodic oxidation, micro-arc oxidation, plasma spraying, ion implantation, electrochemical deposition, sol-gel, friction stir treatment, etc.
Laser surface modification controls the accuracy characteristics of the implanted surface with high efficiency, no pollution, and low material consumption. It is widely applied to preparing periodic micro/nanostructures on the surface of several materials, providing a new idea for the surface modification of metal materials. Unlike the thermal effect caused by molecular vibration induced by a long-wavelength laser, the femtosecond laser has a very low pulse width. Low pulse energy can obtain high peak power, trigger multiphoton absorption and achieve material removal. The thermal effect in femtosecond laser processing can be ignored and the spatial selective manipulation of microstructure can be realized. Femtosecond lasers are suitable for quasi three-dimensional machining of all materials with high machining resolution. Femtosecond lasers are widely used in the preparation of biological functional surfaces.
Body fluid detection provides an early specific indicator for assessing the health status. It requires stable detection method, high sensitivity, and reproducibility. Researchers have prepared femtosecond laser-induced periodic micro-nanostructures on the surface of titanium alloy. Surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) detection substrates (Figs. 6 and 8) were used for glucose detection and in vitro spectral monitoring of protein. The spectral peak intensity has good linearity with the ion concentration to be detected, and the limit of detection (LOD) concentration and sensitivity is good. Thus, the SERS-SEF double enhancement substrate was prepared using the femtosecond laser for urine glucose detection (Fig. 9). The Raman and fluorescence enhancement factors were 7.85×105 and 14.32, respectively, and the LOD was 14.4 mol/L. Additionally, titanium alloy detection substrate was prepared using femtosecond laser-induced surface periodic micro/nanostructure, providing a new idea for body fluid spectral selection and multitarget recognition and detection.
The surface morphology of materials is an important factor affecting the cell behavior on the surface of implants. It is directly related to the subsequent cell proliferation and osteogenic differentiation and is essential in the success or failure of the implant quality scheme. In the study of femtosecond laser-induced surface microstructure regulating the surface cell behavior of clinical implants, laser-induced periodic surface structure (LIPSS), nanopillars (NPs), microgroove, and micropore structure were studied more, among which LIPSS performs well. Femtosecond laser-induced LIPSS is conducive to cell adhesion and serves as a signal to regulate cell migration in a specific direction and stimulate cell proliferation and differentiation (Figs. 11 and 13). One-top femtosecond laser direct writing layered or composite periodic micro/nanostructure of degradable magnesium alloy realizes the increase of cell adhesion on the surface of degradable magnesium alloy, induces cell anisotropic migration and promotes osteoblast bone integration, which provides a new scheme for the clinical application of degradable magnesium alloy.
Presently, the mechanism of microstructure realizing antibacterial function is that the contact/suspension interaction between bacteria and microstructure leads to cell body rupture (Fig. 15) and surface superhydrophobic inhibition of bacterial biofilm formation (Fig. 16). The femtosecond laser induced micro/nanostructured on the surface of titanium alloy not only realized the inhibition of bacteria, such as E.coli, S.aureus, P.gingivalis, which are common in the oral clinic, but also obtained the selective inhibition of colonies (Fig. 18). Furthermore, its surface is nontoxic to cells, which provides an effective method to avoid implant infection and inflammation.
Presently, due to the lack of clear biomedical theoretical guidance in the early design of femtosecond laser surface micro/nanostructure technology, the functional effectiveness of micro/nanostructure depends on subsequent experimental verification. Additionally, the simulation and verification environment of the micro/nanostructure function surfaces is relatively single, such as for single-cell/bacterial behavior, signal transmission, and interaction between multiple cells/bacteria, monitoring and regulation of cell/bacterial behavior on a long time scale need to be further studied.
