Review on cryogenic assisted micro-machining of soft polymer: An emphasis on molecular physics, chamber design, performance analysis and sustainability

Abstract

Over the last few years, soft polymers have found enormous applications in biomedical sector due to their unique material properties. It is of importance to obtain various micro-features on soft biocompatible polymers, to place various sensing elements in wearable sensors and to enable their functionality as that of human tissue or organ. Mechanical micro-machining technique has been widely adopted for micro-feature generation in metallic materials and ceramics. Considering the benefits of mechanical micro-machining techniques such as high geometrical control, shape versatility and highly finished surface, these processes put forward their candidature very strongly for micro-machining of soft polymers too. However, cryogenic assistance is inevitable during mechanical micro-machining of such materials as they do not offer enough resistance for proper shearing on room temperature. It has been observed from the reviewed literature that the mechanical micro-machining of soft polymers has not been established properly and a very few researchers world-wide has adopted the process sometimes. There is no continuous adoption of the process over the years for soft polymers. It is of importance, therefore, to provide a streamlined review of the process where integration between controllable factors of cryogenic technology (e.g. cryogens, delivery methods, temperature control) and mechanical micro-machining elixirs (e.g. size effect) could be put forward for successful application of method for micro-feature generation in soft polymers. This paper reviews the state-of-art developments in cryogenic technology development and its feasible application in soft polymer machining. A brief review of challenges in mechanical micromachining techniques and potential benefits of micromachining of soft polymer under cryogenic environment is presented. The physics of soft polymer behaviour during cryogenic assisted cooling around glass transition temperature (Tg) has been discussed in detail. Although few results were obtained with the available literatures on cryogenic set up, this paper provides directional information by highly emphasizing on the material and design requirement for developing cryogenic chamber for machining soft polymer. The effect of cryogenic machining on process performance characteristics of polymer is discussed and compared with dry condition. A broad assessment on sustainability in cryogenic machining of soft materials in terms of environment, economic and social perspective are outlined in comparison to other previously employed microfabrication process (e.g. LIGA, micro-moulding and embossing) in order to attract the research community for continuous development and use of the process.