Lithography

Micro/nanofabrication technologies have been recently developed as the most interesting methodology used to produce an ordered array of micro/nanopores on the surfaces of silicon. Conventional methods have drawbacks such as broad size distribution of pores, poor mechanical strength, and poor chemical stability. Micro/nanofabrication techniques are widely used to overcome these problems. One example is the work of Desai et al. in which biocapsules with controlled pores of about 7 nm were developed. The authors established that these nanoporous membranes allow exchange of nutrients, waste products, and therapeutic proteins, and that they are biocompatible, providing immune support to cells. In addition, they used their fabricated membrane for an implantable pancreas and oral drug delivery formulation [114,115].

### Focused Ion Beam Etching

Polymeric membranes with a well-ordered array of pore structures and a size range of about 100 nm with a thickness of 1–5 μm have been able to be prepared. The membranes formed with this technique are called nanosieves and are also effectively achieved by photolithography. An excellent example of this method has been prepared by Tong et al. which has included an ordered structure of a cylindrical membrane of diameter 25 nm [116].
