Bioinspired Microstructured Polymer Surfaces with Antireflective Properties.

Over the years, different approaches to obtaining antireflective surfaces have been explored, such as using index-matching, interference, or micro- and nanostructures. Structural super black colors are ubiquitous in nature, and biomimicry thus constitutes an interesting way to develop antireflective surfaces. Moth-eye nanostructures, for example, are well known and have been successfully replicated using micro- and nanofabrication. However, other animal species, such as birds of paradise and peacock spiders, have evolved to display larger structures with antireflective features. In peacock spiders, the antireflective properties of their super black patches arise from relatively simple microstructures with lens-like shapes organized in tightly packed hexagonal arrays, which makes them a good candidate for cheap mass replication techniques. In this paper, we present the fabrication and characterization of antireflective microarrays inspired by the peacock spider's super black structures encountered in nature. Firstly, different microarrays 3D models are generated from a surface equation. Secondly, the arrays are fabricated in a polyacrylate resin by super-resolution 3D printing using two-photon polymerization. Thirdly, the resulting structures are inspected using a scanning electron microscope. Finally, the reflectance and transmittance of the printed structures are characterized at normal incidence with a dedicated optical setup. The bioinspired microlens arrays display excellent antireflective properties, with a measured reflectance as low as 0.042 ± 0.004% for normal incidence, a wavelength of 550 nm, and a collection angle of 14.5°. These values were obtained using a tightly-packed array of slightly pyramidal lenses with a radius of 5 µm and a height of 10 µm.

Consumer-Grade Inkjet Printer for Versatile and Precise Chemical Deposition.

Two simple, mechanical modifications are introduced to a consumer-grade inkjet printer to greatly increase its applicability. First, roller isolation bars are added to unlock multiple prints on the same substrate without smearing. This enables printing on a diverse set of substrates (rigid, elastic, liquid, granular, and sticky). Second, spring loadings are added to increase the print precision up to 50-fold, which facilitates alignment to a pre-patterned substrate or between successive prints. Utilizing the expanded substrate compatibility and the increased print precision, we explore tunable loading of drug combinations into microdevices. This loading method has promising applications within point-of-care personalized medication. Furthermore, we show how inkjet printers with array-type printheads (in our case, 6 x 90 nozzles) allow for quasi-simultaneous loading of reactants into microfluidic systems. The ability to do a quasi-simultaneous introduction of chemicals may be particularly useful for studies of rapidly reacting systems of three or more reactants, where premature introduction can shift the initial conditions from the intended. We believe that our modifications to an affordable system will inspire researchers to explore the possibilities of inkjet printing even further.

Orally ingestible medical devices for gut engineering.

Orally ingestible medical devices provide significant advancement for diagnosis and treatment of gastrointestinal (GI) tract-related conditions. From micro- to macroscale devices, with designs ranging from very simple to complex, these medical devices can be used for site-directed drug delivery in the GI tract, real-time imaging and sensing of gut biomarkers. Equipped with uni-direction release, or self-propulsion, or origami design, these microdevices are breaking the barriers associated with drug delivery, including biologics, across the GI tract. Further, on-board microelectronics allow imaging and sensing of gut tissue and biomarkers, providing a more comprehensive understanding of underlying pathophysiological conditions. We provide an overview of recent advances in orally ingestible medical devices towards drug delivery, imaging and sensing. Challenges associated with gut microenvironment, together with various activation/actuation modalities of medical devices for micromanipulation of the gut are discussed. We have critically examined the relationship between materials-device design-pharmacological responses with respect to existing regulatory guidelines and provided a clear roadmap for the future.

Mapping the transition to superwetting state for nanotextured surfaces templated from block-copolymer self-assembly.

Adding roughness to hydrophilic surfaces is generally expected to enhance their wetting by water. Indeed, global free energy minimization predicts decreasing contact angles when roughness factor or surface energy increases. However, experimentally it is often found that water spreading on rough surfaces is impeded by pinning effects originating from local free energy minima; an effect, largely neglected in scientific literature. Here, we utilize Laplace pressure as a proxy for these local minima, and we map the transition to a superwetting state of hydrophilic nano-textured surfaces in terms of surface chemistry and texture geometry. We demonstrate the effect for polymer model surfaces templated from block-copolymer self-assembly comprising dense, nano-pillar arrays exhibiting strong pinning in their pristine state. By timed argon plasma exposure, we tune surface chemistry to map the transition into the superwetting state of low contact angle, which we show coincide with the surface supporting hemiwicking flow. For the near-ideal model surfaces, the transition to the superwetting state occurs below a critical material contact angle of ∼50°. We show that superwetting surfaces possess anti-fogging properties, and demonstrate long term stability of the superwetting effect by coating the nanotextured surfaces with ∼10 nm thin films of either tungsten or silica.

Generation of micro-droplet arrays by dip-coating of biphilic surfaces; the dependence of entrained droplet volume on withdrawal velocity.

Droplet array chips were realized using an alignment-free fabrication process in silicon. The chips were textured with a homogeneous nano-scale surface roughness but were partially covered with a self-assembled monolayer of perfluorodecyltrichlorosilane (FDTS), resulting in a super-biphilic surface. When submerged in water and withdrawn again, microliter sized droplets are formed due to pinning of water on the hydrophilic spots. The entrained droplet volumes were investigated under variation of spot size and withdrawal velocity. Two regimes of droplet formation were revealed: at low speeds, the droplet volume achieved finite values even for vanishing speeds, while at higher speeds the volume was governed by fluid inertia. A simple 2D boundary layer model describes the behavior at high speeds well. Entrained droplet volume could be altered, post-fabrication, by more than a factor of 15, which opens up for more applications of the dip-coating technique due to the significant increase in versatility of the micro-droplet array platform.

Spatial Control of Condensation on Chemically Homogeneous Pillar-Built Surfaces.

The random nature of dropwise condensation impedes spatial control hereof and its use for creating microdroplet arrays, yet here we demonstrate the spatial control of dropwise condensation on a chemically homogeneous pillar array surface, yielding ∼8000 droplets/mm2 under normal atmospheric pressure conditions. The studied pillar array surface is defined by photolithography and etched in silicon by deep reactive ion etching. Subsequently, the surface is covered with a self-assembled monolayer of perfluorodecyltrichlorosilane (FDTS) to render the surface hydrophobic. To obtain a perfect droplet array, with one droplet per pillar, we exploit a phenomenon where the water vapor flux is focused on the apexes of surface asperities by diffusion while matching the nucleation point density to the array dimensions. Matching is here achieved through the variation of interpillar distance and vapor flow conditions.