Elsevier

Applied Surface Science

Volume 364, 28 February 2016, Pages 37-44
Applied Surface Science

Hierarchically structured self-supported latex films for flexible and semi-transparent electronics

https://doi.org/10.1016/j.apsusc.2015.12.088Get rights and content

Highlights

  • Transparent self-supported latex films were fabricated by a peel-off process.

  • Various template substrates were used for creating e.g. hierarchically structured latex films.

  • Ultra-thin and semi-transparent conductive gold electrodes were evaporated on the latex films.Electrochemical experiments were carried out to verify the applicability of the electrodes.

Abstract

Different length scale alterations in topography, surface texture, and symmetry are known to evoke diverse cell behavior, including adhesion, orientation, motility, cytoskeletal condensation, and modulation of intracellular signaling pathways. In this work, self-supported latex films with well-defined isotropic/anisotropic surface features and hierarchical morphologies were fabricated by a peel-off process from different template surfaces. In addition, the latex films were used as substrates for evaporated ultrathin gold films with nominal thicknesses of 10 and 20 nm. Optical properties and topography of the samples were characterized using UV–vis spectroscopy and Atomic Force Microscopy (AFM) measurements, respectively. The latex films showed high-level transmittance of visible light, enabling the fabrication of semi-transparent gold electrodes. Electrochemical impedance spectroscopy (EIS) measurements were carried out for a number of days to investigate the long-term stability of the electrodes. The effect of 1-octadecanethiol (ODT) and HS(CH2)11OH (MuOH) thiolation and protein (human serum albumin, HSA) adsorption on the impedance and capacitance was studied. In addition, cyclic voltammetry (CV) measurements were carried out to determine active medicinal components, i.e., caffeic acid with interesting biological activities and poorly water-soluble anti-inflammatory drug, piroxicam. The results show that the fabrication procedure presented in this study enables the formation of platforms with hierarchical morphologies for multimodal (optical and electrical) real-time monitoring of length-scale-dependent biomaterial-surface interactions.

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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