2, National University of Singapore, Singapore, , Singapore
Membranes are ubiquitously used but today’s microporous membranes have random pore morphology and tortuous paths which may not suit certain applications. Recent demands in purification processes, cell biology research, and stenciling applications are driving the development of new classes of membranes with very uniform pore architecture (size, distribution, shape, order, density) and straight pore channels.
A plethora micro-nanofabrication methods which enable excellent morphological control have been reported. Amongst the traditional top-down approaches, mold-based imprinting and replica molding techniques are widely adopted for replicating highly dense and ordered pattern from a prefabricated mold onto target material at elevated temperature and pressure or under ultraviolet radiation. However, imprinting process typically leaves behind thin residual layer which requires complex and laborious etching procedures for its removal.
The last two decades have witnessed significant progress in soft lithography which is well known for its remarkable simplicity and good patterning fidelity. Micromolding in Capillaries (MIMIC) is a soft lithography variant capable of generating numerous residue-layer free polymeric structures with ordered dimensions from ~1 μm to >100 μm. The significance of MIMIC process is the in-situ through-thickness pore formation without the need for complex procedures and expensive post etching processes. Instead, conformal contact between the patterned mold and support substrate, and the ensuing capillary flow into the formed channel network are important governing factors. As a result, MIMIC has been increasingly explored as a membrane fabrication technique but like many other microfabrication methods, it is inherently batch-based with manual steps.
To facilitate the transition from batch to continuous production of residue-layer free membranes with precise pore architecture, a novel roll-to-roll (R2R) platform was designed and developed based on MIMIC. The core idea of the R2R platform is to adopt a roller comprises a series of flexible and reusable patterned molds, and to allow each fabrication step to be automated with precise positioning and reproducible force control. The versatile process can also fabricate multilevel, multiscale patterned structures that are difficult to achieve with existing fabrication methods. Complex structures such as ordered porous membranes with integrated nanoporous top layer and double-side surface patterned membranes will be shown as examples.
As a production platform, the first-of-its-kind R2R system offers time, reproducibility and economic incentives over slower, non-automated MIMIC process carried out in batch or, over laborious, expensive post etching processes. The capillary driven, room temperature and solvent-free process is also comparatively a more sustainable printing process for potential scale up production of a number of membrane products.