Bonelike silicon improves interaction with soft tissue.

Chemists at the University of Chicago in collaboration with other researchers at Northwestern University have developed the first skeleton-like silicon spicules ever prepared via chemical processes. This approach, they claim, could improve the integration of medical devices with biological systems.

Fig. 1 – This 3D silicon mesostructure measures approximately 200 nanometers across its narrowest dimension, and is designed to be integrated with biological systems. (Credit: Bozhi Tian Group)
Using bone formation as a guide, the synthetic material developed from silicon shows a potential for improving interaction between soft tissue and hard materials, and seems to enhance soft tissue function.

In describing their method for the synthesis and fabrication of 3D semiconductors that are mesocopic, an intermediate between nanometer and macroscopic scales, the researchers said that their technology creates an opportunity to build electronics for enhanced sensing and stimulation at bio-interfaces. The team achieved three advances in the development of semiconductor and biological materials.

The first was the demonstration, by strictly chemical means, of 3D lithography. Existing lithographic techniques create features over flat surfaces. The laboratory system mimics the natural reaction-diffusion process that leads to symmetry-breaking forms in nature: the grooved and notched form of a bee stinger, for example. The team developed a pressure modulation synthesis, to promote the growth of silicon nanowires and to induce gold-based patterns in the silicon. Gold acts as silicon’s growth catalyst. By repeatedly increasing and decreasing the pressure on their samples, the researchers were able to control the gold’s precipitation and diffusion along the silicon’s faceted surfaces. This utilizing of deposition-diffusion cycles can be applied to synthesizing more complex 3D semiconductors, they explained. (See Figure 1)

3D Silicon Etching

The semiconductor industry uses wet chemical etching with an etch-resist to create planar patterns on silicon wafers. Portions of the wafer masked with thin film physically block the etching from being carried out except on the open surface areas.

A second advance the group developed concerned a novel chemical method that instead depends upon the ability of gold atoms to trap silicon-carrying electrons to selectively prevent the etching.

Surprisingly, the researchers found that even a sparse cover of gold atoms over the silicon matrix would prevent etching from occurring in their proximity. This method also applies to the 3D lithography of many other semiconductor compounds, they said.

Finally, the third advance involved the discovery that the synthetic silicon spicules displayed stronger interactions with collagen fibers, a skin-like stand-in for biological tissue, than with currently available silicon structures. They inserted the synthetic spicules and the other silicon structures into collagen fibers, then pulled them out using an atomic force microscope to measure the force needed to accomplish this action.

The spicules easily penetrated the collagen, then became deeply rooted, much like a bee stinger in human skin.

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