Introduction: The interaction between technology and human body mechanism has been an important topic of studies for decades. Significant research has been conducted to increase the controllability of sensors and bionic implanted devices in human bodies. A good candidate for a material for such biosensors or electro-biomedical devices is silicon, which is not biocompatible in its pure form[1]. In fact, any Si-based bionic device needs to be packaged in a biocompatible material. The challenge is developing a new technology for the silicon to work well as a biomaterial. This paper studies how modifying the surface of silicon using laser nano texturing could increase its biocompatibility. The presented results display a promising improvement in the biocompatibility of silicon for production of biomedical devices such as sensors, bio-NEMS, and nano-biomaterial fabrications.
Materials and Methods: In this study, silicon wafers with orientation of <100> were used. The surface of these wafers were modified using a nanosecond Nd:YAG pulsed laser system, with wavelength of 1064 nm. The laser created a simple line pattern above the ablation threshold at a sub-micro scale. For different samples, the laser would repeat the lines for 1, 2, and 3 overlaps. The frequency was kept constant at 100 KHz, and at an average power of 13.8 W. The scanning speed of the laser was set to 100 mm/s. Once these samples were analyzed under field emission scanning electron microscopy (FESEM), and reviewed with energy-dispersive x-ray spectroscopy (EDX), they were put into simulated body fluid (SBF) for 4 weeks in an incubator at 36.5°C (human body temperature). After this period had elapsed, the samples were taken out and inspected with FESEM and EDX again.
Results and Discussion: Induced laser patterns created a rough surface that increases cell attachment on silicon. The displayed results in Fig. 1 (a, b, c) shows the samples with a 0.025 mm line spacing possessed a nanoporous structure. Once the samples were soaked in SBF (Fig. 1 (d, e, f)), traces of bone-like apatite including calcium, potassium, and chlorine were found on the surface of these structures as seen in Fig 2(a).
The increased surface area due to the roughness change enabled a higher surface energy which results in higher cell attachment rate. It was found that a greater amount of bone-like apatite developed on the higher number of overlaps. Surface roughness can enhance the absorption of light both by multiple reflections in microcavities and by variation in the angle of incidence. Laser-induced periodic surface structures may enhance absorption of laser energy via generation of surface electromagnetic waves[2]. The initial pattern with one overlap created a roughness, which allowed more light absorption. By increasing the overlap number to 2 and 3, the contact surface will increase further thus allowing more apatite to induce on the silicon. The effects of number of overlaps on roughness and light absorption of silicon can be seen in Fig. 2(b).
Conclusion: In this study, the effects of changing the number of overlaps on biocompatibility of silicon were investigated. The results show the apatite inducing ability increases with higher number of overlaps. Hence, with minor surface modifications silicon can become a biocompatible material. Consequently, silicon can potentially be used in fabrication of bionic devices, biosensors and biomaterial applications.
Natural Sciences & Engineering research Council of Canada (NSERC); New Brunswick Innovation Foundation (NBIF)
References:
[1] Lorraine Buckberry, S. B. (1999). Porous Silicon as a Biomaterial. The Institute of Materials. Materials World.
[2] Vorobyev, A. Y., & Guo, C. (2005). Enhanced absorptance of gold following multipulse femtosecond laser ablation. The Institute of Optics. New York: University of Rochester.