Stephen A. Morin’s research while affiliated with University of Nebraska at Lincoln and other places

The surface chemistry and microstructure of a material determine its macroscopic (e.g., wettability, adhesion) and microscopic (e.g., catalytic activity, molecular speciation) interfacial properties. Synthetic strategies applicable to the design and fabrication of well-defined interfaces, including prototypical systems based on self-assembled monolayers, polymer coatings, and microtextured materials, have been reported extensively by the surface science community. These systems are typically supported on mechanically rigid/planar supports, providing stable surfaces ideally suited for applications demanding static and predictable interfacial properties. Emergent technologies, including soft actuators and robotics, stretchable sensors and electronics, and surface fluidics, demand mechanically compliant, elastic support substrates and dynamically adaptive chemical/mechanical surface properties that are inaccessible in traditional systems. Accordingly, there have been considerable efforts to design, synthesize, and characterize new varieties of soft, stimuli-responsive surfaces. This article provides an overview of state-of-the-art stimuli-responsive soft surfaces, emphasizing those with mechanical and chemical control vectors. Specifically, the synthetic strategies, dynamic properties for adaptive response, prospective applications, existing challenges, and future opportunities for these new stimuli-responsive surfaces are covered.

Traditional circuit board fabrication schemes are not directly applicable to the production of flexible, multi‐material circuits. This article reports a technique, microfluidic directed material patterning, which combines soft microfluidic stamps and low‐temperature solution‐phase deposition to generate multi‐material circuits on flexible, non‐planar polymeric supports. Specifically, metallic and semiconductive traces are combined on commodity plastic films to yield functional photosensitive circuits that can be used in the spectrophotometric detection and concentration measurement of microdroplets on 3D “e‐plates.” The photoresistive material cadmium sulfide is used in these circuits because it is suitable for visible light detection and it can be deposited directly from aqueous solutions following established bath deposition procedures. This method can produce colorimetric devices capable of quantifying micromolar concentrations of Allura Red in microdroplets of Kool‐Aid. This technique presents the opportunity for producing single‐use or low‐use disposable/recyclable devices for flexible 3D sensors and detectors following a convenient, low‐waste fabrication scheme. The general capabilities of this approach, in terms of substrate geometry and device layout (e.g., the number, area, and pattern of photoresistive elements), can be applied to the design and manufacture of more intricate, multiplexed devices supportive of advanced and/or specialized functions that go beyond those reported in this initial demonstration.

The capability to produce suture material using three‐dimensional (3D) printing technology may have applications in remote health facilities where rapid restocking of supplies is not an option. This is a feasibility study evaluating the usability of 3D‐printed sutures in the repair of a laceration wound when compared with standard suture material. The 3D‐printed suture material was manufactured using a fused deposition modelling 3D printer and nylon 3D printing filament. Study participants were tasked with performing laceration repairs on the pigs' feet, first with 3‐0 WeGo nylon suture material, followed by the 3D‐printed nylon suture material. Twenty‐six participants were enrolled in the study. Survey data demonstrated statistical significance with how well the 3D suture material performed with knot tying, 8.9 versus 7.5 ( p = 0.0018). Statistical significance was observed in the 3D‐printed suture's ultimate tensile strength when compared to the 3‐0 Novafil suture (274.8 vs. 199.8 MPa, p = 0.0096). The 3D‐printed suture also demonstrated statistical significance in ultimate extension when compared to commercial 3‐0 WeGo nylon suture (49% vs. 37%, p = 0.0215). This study was successful in using 3D printing technology to manufacture suture material and provided insight into its usability when compared to standard suture material.

Hydrogels have emerged as prototypical stimuli‐responsive materials with potential applications in soft robotics, microfluidics, tissue engineering, and adaptive optics. To leverage the full potential of these materials, fabrication techniques capable of simultaneous control of microstructure, device architecture, and interfacial stability, that is, adhesion of hydrogel components to support substrates, are needed. A universal strategy for the microfabrication of hydrogel‐based devices with robust substrate adhesion amenable to use in liquid environments would enable numerous applications. This manuscript reports a general approach for the facile production of covalently attached, ordered arrays of microscale hydrogels (microgels) on silicone supports. Specifically, silicone‐based templates are used to: i) drive mechanical assembly of prepolymer droplets into well‐defined geometries and morphologies, and ii) present appropriate conjugation moieties to fix gels in place during photoinitiated crosslinking via a “graft from” polymerization scheme. Automated processing enabled rapid microgel array production for characterization, testing, and application. Furthermore, the stimuli‐responsive microlensing properties of these arrays, via contractile modulated refractive index, are demonstrated. This process is directly applicable to the fabrication of adaptive optofluidic systems and can be further applied to advanced functional systems such as soft actuators and robotics, and 3D cell culture technologies.

A general approach to increase the adhesion of metal films to commodity plastic substrates using a metal-chelating polymer, polyethyleneimine, in conjunction with patterned electroless deposition is described. This general fabrication...

The effect of anisotropic surface roughness on the spontaneous transport of droplets on chemical wettability gradients has not been investigated. Understanding the details of this process has the potential to unlock new fluid handling functionality critical to the development of next-generation surfaces with intelligent control capabilities. Herein, the fabrication of chemical gradients with mechanically tunable anisotropic microtopography (microwrinkles with directional roughness) is described and the use of these surfaces in programable microdroplet transport is reported. In particular, the interplay between chemical gradient intensity, microwrinkle orientation, and droplet velocity/trajectory was investigated, enabling the rational synthesis of surface fluidic systems capable of mechanically programmable 2D droplet manipulations, vertical droplet transport, and droplet combination. These findings highlight the sophisticated capabilities of mechanically switchable droplet handling systems and demonstrate new avenues for designing intelligent materials with programable transport properties for potential use in surface/microfluidics, water harvesting, energy generation, bioanalysis, and microreactor design.

Natural particles possess varying degrees of surface heterogeneities. Although the impact of collector surface heterogeneity on particle deposition is more or less established, the impact of particle surface heterogeneity on the deposition of colloids in porous media is not well studied. In this work, Janus SiO2 microparticles (Janus SiO2 MPs) were engineered by partially covering SiO2 microspheres (500 nm in diameter) with positively charged poly(L-lysine) (PLL) as model particles to study the impact of particle surface heterogeneity on the deposition. The surface heterogeneity of Janus SiO2 MP was confirmed using Kelvin probe force microscopy. Up to 4–5 original SiO2 microspheres tended to assemble in the solution to form relatively stable clusters, with an isoelectric point slightly higher than pH 7. Compared with unmodified SiO2 microspheres, Janus SiO2 MPs had a slower attachment rate on silicon dioxide surfaces. While the deposition of Janus SiO2 MPs onto silicon dioxide surfaces was observed at pH values of 3 and 7, no deposition was observed at pH 9. No deposition was observed for Janus SiO2 MPs onto aluminum oxide surfaces for pH values ranging from 3 to 9, even under an overall favorable attractive condition. This work found that the deposition of colloids is more sensitive to particle surface heterogeneity properties than to the overall surface potential of the colloids.

Adaptive materials with tunable superhydrophobic surfaces promise to impact a range of fluid handling technologies; however, adaptive superhydrophobic materials remain difficult to fabricate, control, and switch rapidly. Here, a versatile method for generating hierarchically structured and adaptive superhydrophobic silicone films for the rational control of surface wettability and droplet adhesion is reported. Specifically, mechanical tension is utilized to manipulate networks of microcracks in nanoporous layers supported on elastomeric silicone films, enabling dynamic modulation of superhydrophobicity and droplet adhesion. The reported mechano-responsive superhydrophobic surfaces are applied to directional droplet shedding and “no-loss” droplet transport and are used to generate artificial “skins” with droplet tweezing capabilities. This approach provides materials with enhanced functionality useful to a range of emergent technologies, including adaptive textiles, biocompatible (wearable) sensors, soft robotics, anti-icing systems, “no-loss” droplet manipulators, and thermal management devices.