Preparation of a self-matting, anti-fingerprint and skin-tactile wood coating via biomimetic self-wrinkling patterns

Inspired by natural wrinkled surfaces, artificial surfaces with biomimetic wrinkled structures have been widely used to improve optical properties, wettability, and antibacterial properties. However, the preparation of wrinkled structures has the disadvantages of long-time consumption and complex processes. Herein, we prepared a self-wrinkling polyurethane-acrylate (PUA) wood coating via biomimetic self-wrinkling patterns by using a light-emitting diode (LED)/excimer/mercury lamp curing system, which was capable of self-matting, anti-fingerprint and skin-tactile performance. By adjusting the irradiation intensity in the curing system, the wavelength (λ) and amplitude (A) of wrinkles on the coating surface were controlled to enhance the coating performance. After curing by the LED, excimer, and mercury lamps at energy intensities of 500, 30, and 300 mW/cm2 respectively, the self-wrinkling coating showed excellent surface performance. The self-wrinkling coating represented low gloss of 4.1 GU at 85°, high hardness of 4H. Interestingly, the coating surface had a high hydrophobicity (104.5°) and low surface energy (29-30 mN/m) and low coefficient (COF) of friction (0.1-0.2), which were consistent with those of the human skin surface. Besides, the wrinkled structure also improved the thermal stability of the coating samples. This study provided a promising technique for the mass production of self-wrinkling coatings that could be used in wood-based panels, furniture, and leather.

Self-Wrinkling in Polyacrylamide Hydrogel Bilayers

Swelling of a gel film attached to a soft substrate can induce surface instability, which results in the formation of highly ordered patterns such as wrinkles and folds. This phenomenon has been exploited to fabricate functional devices and rationalize morphogenesis. However, obtaining centimeter-scale patterns without immersing the film in a solvent remains challenging. Here, we show that wrinkles with wavelengths of up to a few centimeters can be spontaneously created during the open-air fabrication of film-substrate bilayers of polyacrylamide (PAAm) hydrogels. When the film of an aqueous pregel solution of acrylamide prepared on the PAAm hydrogel substrate undergoes open-air gelation, hexagonally packed dimples initially emerge on the surface, which later evolve into randomly oriented wrinkles. The formation of such self-organized patterns can be attributed to the surface instability resulting from autonomous water transport in the bilayer system during open-air fabrication. The temporal evolution of the patterns can be ascribed to an increase in overstress in the hydrogel film due to continued water uptake. The wrinkle wavelength can be controlled in the centimeter-scale range by adjusting the film thickness of the aqueous pregel solution. Our self-wrinkling method provides a simple mechanism for the generation of swelling-induced centimeter-scale wrinkles without requiring an external solvent, which is unachievable with conventional approaches.

UV-curable self-matting waterborne polyurethane acrylate coating via self-wrinkled surface during curing in open-air

This study developed and evaluated a series of ultraviolet (UV) curable self-matting waterborne polyurethane acrylate (UV-WPUA) coatings based on the self-wrinkled surface during UV-curing in the open-air. This method is simple, efficient, eco-friendly, and it does not require complicated or expensive equipment. The FT-IR spectrum indicates that the peaks of C[double bond, length as m-dash]C in UV-WPUA have disappeared after UV irradiation. The gloss value of the UV-WPUA cured film can be affected by the wrinkles on the surface of the film and adjusted it by controlling the content of the photoinitiator in the liquid coating, since the content influences the dimensions of the wrinkles. As the height of the wrinkle increased, the gloss value of the UV-WPUA cured film decreased, and when the incident angles are 20° and 60°, the gloss values are less than 3 GU and 5 GU, respectively. Moreover, the cured film has a maximum water contact angle of 109°, which is affected by wrinkles and positively correlated with the surface roughness of the film. Furthermore, the cured film has excellent properties of thermal stability, tensile strength, pencil hardness, cross-cut adhesion and resistance to abrasion properties, making it well suited for the furniture, leather, and textile applications.

Dissociative Carbamate Exchange Anneals 3D Printed Acrylates

Relative to other additive manufacturing modalities, vat photopolymerization (VP) offers designers superior surface finish, feature resolution, and throughput. However, poor interlayer network formation can limit a VP-printed part's tensile strength along the build axis. We demonstrate that the incorporation of carbamate bonds capable of undergoing dissociative exchange reactions provides improved interlayer network formation in VP-printed urethane acrylate polymers. In the presence of dibutyltin dilaurate catalyst, the exchange of these carbamate bonds enables rapid stress relaxation with an activation energy of 133 kJ/mol, consistent with a dissociative bond exchange process. Annealed XY tensile samples containing a catalyst demonstrate a 25% decrease in Young's modulus, attributed to statistical changes in network topology, while samples without a catalyst show no observable effect. Annealed ZX tensile samples printed with layers perpendicular to tensile load demonstrate an increase in elongation at break, indicative of self-healing. The strain at break for samples containing a catalyst increases from 33.9 to 56.0% after annealing but decreases from 48.1 to 32.1% after annealing in samples without a catalyst. This thermally activated bond exchange process improves the performance of VP-printed materials via self-healing across layers and provides a means to change Young's modulus after printing. Thus, the incorporation of carbamate bonds and appropriate catalysts in the VP-printing process provides a robust platform for enhancing material properties and performance.

Integrated dynamic wet spinning of core-sheath hydrogel fibers for optical-to-brain/tissue communications

Hydrogel optical light-guides have received substantial interest for applications such as deep-tissue biosensors, optogenetic stimulation and photomedicine due to their biocompatibility, (micro)structure control and tissue-like Young's modulus. However, despite recent developments, large-scale fabrication with a continuous synthetic methodology, which could produce core-sheath hydrogel fibers with the desired optical and mechanical properties suitable for deep-tissue applications, has yet to be achieved. In this study, we report a versatile concept of integrated light-triggered dynamic wet spinning capable of continuously producing core-sheath hydrogel optical fibers with tunable fiber diameters, and mechanical and optical propagation properties. Furthermore, this concept also exhibited versatility for various kinds of core-sheath functional fibers. The wet spinning synthetic procedure and fabrication process were optimized with the rational design of the core/sheath material interface compatibility [core = poly(ethylene glycol diacrylate-co-acrylamide); sheath = Ca-alginate], optical transparency, refractive index and spinning solution viscosity. The resulting hydrogel optical fibers exhibited desirable low optical attenuation (0.18 ± 0.01 dB cm⁻¹ with 650 nm laser light), excellent biocompatibility and tissue-like Young's modulus (<2.60 MPa). The optical waveguide hydrogel fibers were successfully employed for deep-tissue cancer therapy and brain optogenetic stimulation, confirming that they could serve as an efficient versatile tool for diverse deep-tissue therapy and brain optogenetic applications.

Drawing High-Definition and Reversible Hydrogel Paintings with Grayscale Exposure

High-definition and arbitrary grayscale hydrogel paintings that can appear reversibly with hydration/dehydration are realized through spatially grayscale exposure. Spatio-temporally grayscale images are used to guide the exposure of a hydrogel processor to dictate the gradient cross-linking density spatially, which thereafter results in the heterogeneity of hydrogels in swelling ratio, mechanical properties, and especially visible light transmittance, leading to swelling-induced patterns by gradient and local visible light scattering difference based on tunable mesh size and microphase separation. The resultant grayscale hydrogel patterns, with visible light transmittance adjustable, are reversible during hydration (in 1-2 s) and dehydration and possess the feature size of 70 μm and more pattern information compared with previous hydrogel patterning. Uniquely, the patterns can be realized not only on the outmost surface of hydrogels as usual but inside. Combining the unique grayscale exposure with three-dimensional printing technology, arbitrary hydrogel patterns that have plenty of details and even vary at different layers are fabricated readily, indicating its broad potential in smart anti-counterfeiting of security field, mechanics, engineering, and many others.

Monomer diffusion into static and evolving polymer networks during frontal photopolymerisation

Frontal photopolymerisation (FPP) is a directional solidification process that converts monomer-rich liquid into crosslinked polymer solid by light exposure and finds applications ranging from lithography to 3D printing. Inherent to this process is the creation of an evolving polymer network that is exposed to a monomer bath. A combined theoretical and experimental investigation is performed to determine the conditions under which monomer from this bath can diffuse into the propagating polymer network and cause it to swell. First, the growth and swelling processes are decoupled by immersing pre-made polymer networks into monomer baths held at various temperatures. The experimental measurements of the network thickness are found to be in good agreement with theoretical predictions obtained from a nonlinear poroelastic model. FPP propagation experiments are then carried out under conditions that lead to swelling. Unexpectedly, for a fixed exposure time, swelling is found to increase with incident light intensity. The experimental data is well described by a novel FPP model accounting for mass transport and the mechanical response of the polymer network, providing key insights into how monomer diffusion affects the conversion profile of the polymer solid and the stresses that are generated during its growth. The predictive capability of the model will enable the fabrication of gradient materials with tuned mechanical properties and controlled stress development.