What are the techniques available for fabrication of 3D structures on the micrometer scale? Explain
Microfabrication techniques have been employed to fabricate integrated cir-cuits (ICs), semiconductors, microelectromechanical systems (MEMS), nanoelectromechanical systems (NEMS), BioMEMS, microfluidic, electromechanical, mechanical, chemical, optical, photonic and multifunctional devices. Nano-technology has borrowed microfabrication techniques for miniaturization science. Some of these techniques have very old origins, not connected to manufacturing, like lithography or etching. Biomedical research, over the past decade, has exploited these microfabrication techniques to fabricate biomaterials in order to study the cell–biomaterial interaction at the micro/nanoscale environment. These approaches can be used to provide model surfaces that would allow scientists to explore the relationship between cells: biomaterial substrate morphology. Biomaterials with controlled morphology can be effectively utilized to induce favourable cell response. In this chapter, a simple, cost-effective technique of ‘soft photolithography’, a combination of photolithography and soft lithography is described. Such a technique has been used to fabricate hydrogel micropatterns of sub-cellular dimensions as fine as 10 μm. These sub-cellular biomaterial micropatterns can be effectively employed to study cell–biomaterial interactions and for their applications in tissue engineering.
Subtractive microfabrication techniques employ removal or etching of a portion of substrate to fabricate microstructures/patterns. Dry etching and wet etching are two types of subtractive microfabrication techniques. (i) Dry etching refers to the removal of material, typically a masked pattern of semiconductor material by exposing the material to a bombardment of ions (usually plasma of reactive gases such as fluorocarbons, oxygen, chlorine, boron trichloride; sometimes with addition of nitrogen, argon, helium, and other gases) that dislodge portions of the material from the exposed surface. (ii) Wet etching is chemical etching performed with a liquid chemical (etchant) instead of plasma
Microfabrication techniques have been used for the creation of Si-based MEAs for neural interfacing, including peripheral nerve electrodes and intracortical electrodes as these techniques enable batch processing, design flexibility, and small, dense features that cannot easily be achieved by hand fabrication. Initially, silicon substrates were used for microfabricated neural interfaces in light of its established fabrication processes, lack of biotoxicity, stiffness sufficient for penetrating the cortical tissue without buckling, and ability to integrate signal processing circuitry directly on-chip . However, mechanically rigid intracortical probes induce large strains on the neural tissue during micromotion of the brain which can lead to tissue damage. This tissue damage is hypothesized to be the cause of the development of an interfering cellular sheath that has been observed to form around neural probes, limiting their viability to only a few months Strain induced on the neural tissue may be reduced with the use of mechanically flexible polymer substrates for intracortical probes It is thought that polymer-based substrates may be preferable to silicon in terms of functional reliability exceeding 10 years.