Polymer Research Institute, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China
Abstract:
Honeycomb-patterned porous films are polymer films with regular pore arrays on their surfaces. Since the pioneering work of François et al. in 1994, in which they used the breath figure (BF) technique to prepare honeycomb films, these highly ordered porous films have been attracting increasing interest in the past decades. Researchers are interested in the well-ordered pore arrays as they show great potential for use in many areas including superhydrophobic materials, photoelectric materials, tissue engineering, biomedicine, gas sensors, micro-reactors, to name just a few. Previous studies in this area have mainly focused on the preparation of porous films with regular microstructures and the effect of polymer architecture and casting conditions (such as temperature, relative humidity, and solvents) on the morphology. During the past two decades, considerable work has been devoted to identifying the mechanism of generation of well-ordered pore arrays during the BF process. Although the exact mechanism of film formation remains unclear, highly ordered honeycomb films can be produced from various polymers or polymer blends. In other words, currently, preparation of honeycomb films is not a major challenge. More recent studies in this area have concentrated on fabricating smart honeycomb films with reversible surface morphologies and/or properties. These smart films possess not only the properties of ordinary honeycomb films but also unique "on-off" switching functions. The research on stimuli-responsive smart honeycomb films is quite interesting in terms of both fundamental research and practical applications. Theoretically, the different scales and regular pore arrays provide an ideal model for studying the surface properties of porous materials under external stimulation, which is helpful in designing structured surfaces with desirable properties. From an application perspective, the "on-off" switching behavior imparts the films with additional functions that show high potential in a wide range of applications such as cell adhesion and controlled release, protein adsorption and separation, controlled drug release, etc. Thus far, honeycomb films with stimuli-responsive reversible surface morphologies, wettability, and fluorescence spectra have been developed, and the stimulus triggers have been mainly concentrated on temperature, pH, light, solvents, and gases. The main focus of this study is to describe the recent advances in smart honeycomb films, including fabrication strategies, triggers, "on-off" switching mechanisms, responsive behaviors, and related applications. Moreover, special attention is given to discussing the advantages and disadvantages of smart honeycomb films based on different triggers and design of smart honeycomb film systems with improved response properties. This study also discusses the challenges that concern the future development of smart honeycomb films and suggests several means of addressing those challenges.
