Hierarchically structured self-supported latex films for flexible and semi-transparent electronics
Graphical abstract
Introduction
The control and monitoring of protein-material and cell-protein-material interactions are important subjects with implications for the biosensor field [1], [2] and the medical field dealing with surgical implant-associated bacterial infections [3], [4], compatibility issues [5], [6], tissue engineering [7], [8], organ transplant rejection [9] and wound healing [10]. In the quickly evolving field of bioelectronics, electronics and biological interfaces are coupled to improve biochemical sensing, tissue characterization, organ monitoring, therapeutics, and diagnostics [11]. Electrical methods are able to detect low concentrations of biological analytes and these methods require no labeling. On the other hand also electrical [12], [13] and optical [14] measurements can be carried out simultaneously provided that transparent or semitransparent conductive electrodes and substrates are used [11], [15], [16].
Traditionally, in vitro cell culture studies have been carried out using flat and clear plastic 2D surfaces [17]. It is, however, well known that on flat and hard substrates, cells behave in a different manner compared to the environments in living tissues [18], [19], [20], [21], [22]. For example, stem cells may differentiate into neurons, osteoblasts or myocytes depending on the stiffness of the substrate [23]. To enhance the well-being of cells and to induce a more in vivo like behavior, the influence of surface topography and in vivo mimicking of 3D features have been studied [17], [18], [24], [25], [26]. Textured surfaces have been fabricated by several methods, often by photolithography and etching [25]. In addition, nanoimprinting [27] and different laser modification techniques have been also used for this purpose [28], [29]. Biodegradable thin films of poly-L-lactic acid [30] and chitosan [31] have been fabricated using soft lithographic techniques by applying the polymer solutions on the template surfaces and by peeling them off after solvent evaporation. Recently, De Rosa et al. described a solvent treatment method for creating pores on flat polystyrene surfaces thereby gaining improved cell function mainly by enabling cells to form 3D aggregates [17]. Strain responsive wrinkling technique was used by Choi et al. to create structured PDMS substrates [32]. Zhang et al. used focused ion beam milling to create regularly patterned gold films with a wide palette of colors without employing any form of chemical modification [33]. Morariu et al. described an electric field-induced sub-100 nm scale structure formation process using polymer bilayers [34].
In this work, we expand our previous work with paper supported latex films [35], [36], [37] and show the fabrication of hierarchically structured self-supported latex films with high-level transmittance of visible light. In addition, the use of the self-supported films as potential substrates for transparent electronics is demonstrated. The films were prepared by a peel-off process from different surfaces. Atomic Force Microscopy (AFM) calibration grids with accurately determined dimensions, glass slides and a more mass-scale compatible roll-to-roll fabricated curtain coated paper substrate were used as templates. The peeled latex films include a characteristic primary structure of a two-component latex coating material [35], [36], [37] and an additional secondary structural feature determined by the surface morphology of the used template substrate. Evaporated gold electrodes were fabricated on the self-supported latex films to enable electrical functions.
Section snippets
Template substrates
Four different AFM calibration grids (models: TGG1, TGZ2, TGT1 and TGX1, NT-MDT, Russia), microscope glass slides (Menzel-Gläser, Thermo scientific, Germany), Polydimethylsiloxane (PDMS) [36] (Wacker, Germany) and a multilayer curtain coated paper [38] were used as model template substrates from which the latex coatings were peeled off.
Coating material
The two component coating latex blend with a weight ratio of 1:1 was prepared by mixing aqueous dispersions of polystyrene particles (HPY83; average particle size
Preparation and topographical characterization of hierarchically structured self-supported films and semi-transparent electronics
Different kinds of template substrates were used for the preparation of the self-supported latex films depending on the hierarchical structure desired. For example, sub-nanometer and nanometer scale features can be prepared by rod-coating the latex blend dispersion on a pigment coated paper substrate. After an IR-treatment, a distinct nanostructured topography with bimodal height distribution and random distribution, depending on the ratio of soft and hard components in the latex blend [42] was
Conclusions
The aim of this work was to fabricate highly transparent films with readily adjustable surface topography using latex blends that have been shown to be suitable substrate materials for both cell growth studies and for fabrication of conducting ultrathin gold films by evaporation. The transparency and good biocompatibility of both the latex substrate and gold enable the use of the substrates in studies where for example the effects of topography and applied surface potential on the adsorption of
Acknowledgments
The author M. P. thankfully acknowledges the financial support from the Åbo Akademi University Endowment. The author A. M. thankfully acknowledges the Graduate School at Åbo Akademi University for the granted research scholarship.
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