A NOVEL MICROFABRICATION TECHNIQUE FOR DEVELOPMENT OF A 3D PYRAMIDAL POROUS MEMBRANE

Abstract

Three-dimensional (3D) microfabrication has a niche role in emerging technologies as essential components of various MEMS and biomedical applications. However, the 3D microfabrication is quite challenging as it requires simultaneous control over fabrication of lateral as well as vertical dimensions of microstructures. To achieve such control, highly localized light exposure in photosensitive materials is sought in existing 3D microfabrication techniques. The exposed regions cross-link and microstructures are fabricated according to pattern of the exposure. However, this strategy is complicated, expensive and slow as it employs point-to-point or layer-by-layer exposure pattern which requires highly sophisticated and expensive equipment. Herein, it is hypothesized that controlling the cross-linking instead of the exposure would help in developing a simple 3D microfabrication technique. The cross-linking in lateral dimensions can be easily controlled by exposing photosensitive material through a binary coded photo-mask (as in photolithography). Nonetheless, controlling the light exposure or cross-linking in vertical dimension is not possible by using a normal photo-mask due to `all-or-none¿ exposure pattern leading to complete cross-linking of the exposed region. Hence, a new strategy involving partial activation (PA) has been developed in this study for controlling cross-linking in both dimensions using photo-mask based exposure system. PA represents a state of material in which slight activation of cross-linking initiators is generated but the amount of such initiators is not enough for inducing cross-linking reaction. In such a state, anisotropic cross-linking can take place which can provide opportunities for 3D microfabrication. Thus, this strategy allows fabrication of 3D microstructures besides retaining the speed and the simplicity of photo-mask exposure scheme. Different types of 3D microstructures have been fabricated by using a single inexpensive photo-mask at high throughput in this project. The purpose of the 3D microfabrication technique in this project is to develop a functional micro-device for sorting and patterning of micro-entities such as cells or beads which is useful for disease diagnosis and treatment. The existing techniques for sorting or patterning of micro-entities are expensive, requires tagging of micro-entities and living cells are subjected to electromagnetic field with unknown implications. Porous membrane presents the simplest option for sorting besides having high throughput performance. However, pore clogging is a common drawback in using porous membrane. Moreover, porous membrane is less suitable for patterning. Here, a unique anti-clogging three-dimensional (3D) pyramidal porous membrane (3DPPM) with multiple functionalities for enhanced performance has been developed by using newly developed 3D microfabrication technique. The 3DPPM consists of an array of funnel-like pores, wherein each pore is surrounded by four 3D pyramidal micro-structures. The design of the 3DPPM developed in this project confers four useful features. 1) It is anti-clogging and therefore allows uninterrupted sorting of micro-entities continuously; 2) Simultaneous sorting and patterning of micro-entities can be achieved; 3) Inhomogeneous cell population of different sizes can be patterned; 4) Bi-directional sorting can be achieved, both in the direction of fluid-flow through the pores as well as that perpendicular to it, for high sorting efficiency.