Photografting of Surface-assembled Hydrogel Prepolymers to Elastomeric Substrates for Production of Stimuli-Responsive Microlens Arrays.

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, i.e., 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 were 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, were 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.

An evaluation of the usability and durability of 3D printed versus standard suture materials.

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.

Spatiotemporal control of calcium carbonate nucleation using mechanical deformations of elastic surfaces.

Biological systems generate crystalline materials with properties and morphologies that cannot be duplicated using synthetic procedures. Developing strategies that mimic the control mechanisms found in nature would enhance the range of functional materials available for numerous technological applications. Herein, a biomimetic approach based on the mechano-dynamic chemistry of silicone surfaces was used to control the rate of heterogeneous CaCO3 nucleation. Specifically, stretching the silicone surface redistributed functional groups, tuning interfacial energy and thus the rate of CaCO3 crystal formation, as predicted by classical nucleation rate laws. We extended this procedure using microrelief patterns to program surface strain fields to spatially control the location of nucleation. The strategies presented herein represent a fundamental departure from traditional bottom-up crystal engineering, where surfaces are chemically static, to them being active participants in the nucleation process controlling the outcome both spatially and temporally.

Crystallization at droplet interfaces for the fabrication of geometrically programmed synthetic magnetosomes.

Biological systems demonstrate exquisite three dimensional (3D) control over crystal nucleation and growth using soft micro/nanoenvironments, such as vesicles, for reagent transport and confinement. It remains challenging to mimic such biomineralization processes using synthetic systems. A synthetic mineralization strategy applicable to the synthesis of artificial magnetosomes with programmable magnetic domains is described. This strategy relies on the compartmentalization of precursors in surfactant-stabilized liquid microdroplets which, when contacted, spontaneously form lipid bilayers that support reagent transport and interface-confined magnetite nucleation and growth. The resulting magnetic domains are polarized and thus readily manipulated using magnetic fields or assembled using droplet-droplet interactions. This strategy presents a new, liquid phase procedure for the synthesis of vesicles with geometrically controlled inorganic features that would be difficult to produce otherwise. The artificial magnetosomes demonstrated could find use in, for example, drug/cargo delivery, droplet microfluidics, and formulation science.

Emergent Soft Lithographic Tools for the Fabrication of Functional Polymeric Microstructures.

Polymeric microstructures (PMs) are useful to a broad range of technologies applicable to, for example, sensing, energy storage, and soft robotics. Due to the diverse application space of PMs, many techniques (e. g., photolithography, 3D printing, micromilling, etc.) have been developed to fabricate these structures. Stemming from their generality and unique capabilities, the tools encompassed by soft lithography (e. g., replica molding, microcontact printing, etc.), which use soft elastomeric materials as masters in the fabrication of PMs, are particularly relevant. By taking advantage of the characteristics of elastomeric masters, particularly their mechanical and chemical properties, soft lithography has enabled the use of non-planar substrates and relatively inexpensive equipment in the generation of many types of PMs, redefining existing communities and creating new ones. Traditionally, these elastomeric masters have been produced from relief patterns fabricated using photolithography; however, recent efforts have led to the emergence of new methods that make use of masters that are self-forming, dynamic in their geometric and chemical properties, 3D in architecture, and/or sacrificial (i. e., easily removed/released using phase changes). These "next generation" soft lithographic masters include self-assembled liquid droplets, microscale balloons, templates derived from natural materials, and hierarchically microstructured surfaces. The new methods of fabrication supported by these unique masters enable access to numerous varieties of PMs (e. g., those with hierarchical microstructures, overhanging features, and 3D architectures) that would not be possible following established methods of soft lithography. This review explores these emergent soft lithographic methods, addressing their operational principles and the application space they can impact.

Surface Molding of Microscale Hydrogels with Microactuation Functionality.

This work describes the fabrication of numerous hydrogel microstructures (µ-gels) via a process called "surface molding." Chemically patterned elastomeric-assembly substrates were used to organize and manipulate the geometry of liquid prepolymer microdroplets, which, following photo-initiated crosslinking, maintained the desired morphology. By adjusting the state of strain during the crosslinking process, a continua of structures could be created using one pattern. These arrays of µ-gels have stimuli-responsive properties that are directly applicable to actuation where the basis shape and array geometry of the µ-gels can be used to rationally generate microactuators with programmed motions. As a method, "surface molding," represents a powerful addition to the soft-lithographic toolset that can be readily applied to the simultaneous synthesis of large numbers of geometrically and functionally distinct polymeric microstructures.

Stretchable Substrates for the Assembly of Polymeric Microstructures.

The directed assembly of micro-/nanoscale objects relies on physical or chemical processes to generate structures that are not possible via self-assembly alone. A relatively unexplored strategy in directed assembly is the "active" manipulation of building blocks through deformations of elastomeric substrates. This manuscript reports a method which uses macroscopic mechanical deformations of chemically modified silicone films to realize the rational assembly of microscopic polymer structures. Specifically, polystyrene microparticles are deposited onto polydimethylsiloxane substrates using microcontact-printing where, through a process that involved stretching/relaxing the substrates and bonding of the particles, they are elaborated into microstructures of various sizes, shapes, symmetries, periodicities, and functionalities. The resulting polymeric microstructures can be released and redeposited onto planar/nonplanar surfaces. When building blocks with different properties (e.g., those with fluorescent and catalytic properties) are used, it is possible to fabricate structures with heterogeneous functionality. This method can be extended to the assembly of numerous micro-/nanoscale building blocks (e.g., colloidal organic/inorganic materials) with rational control over the size, shape, and functionality of the product. As a strategy, the use of substrate deformations to enable the micromanipulation and fabrication of a potentially diverse set of assemblies represents a powerful tool useful to, for example, nanotechnology and micromanufacturing.

Adaptive use of bubble wrap for storing liquid samples and performing analytical assays.

This paper demonstrates that the gas-filled compartments in the packing material commonly called "bubble wrap" can be repurposed in resource-limited regions as containers to store liquid samples, and to perform bioanalyses. The bubbles of bubble wrap are easily filled by injecting the samples into them using a syringe with a needle or a pipet tip, and then sealing the hole with nail hardener. The bubbles are transparent in the visible range of the spectrum, and can be used as "cuvettes" for absorbance and fluorescence measurements. The interiors of these bubbles are sterile and allow storage of samples without the need for expensive sterilization equipment. The bubbles are also permeable to gases, and can be used to culture and store micro-organisms. By incorporating carbon electrodes, these bubbles can be used as electrochemical cells. This paper demonstrates the capabilities of the bubbles by culturing E. coli, growing C. elegans, measuring glucose and hemoglobin spectrophotometrically, and measuring ferrocyanide electrochemically, all within the bubbles.

Screw dislocation driven growth of nanomaterials.

Nanoscience and nanotechnology impact our lives in many ways, from electronic and photonic devices to biosensors. They also hold the promise of tackling the renewable energy challenges facing us. However, one limiting scientific challenge is the effective and efficient bottom-up synthesis of nanomaterials. We can approach this core challenge in nanoscience and nanotechnology from two perspectives: (a) how to controllably grow high-quality nanomaterials with desired dimensions, morphologies, and material compositions and (b) how to produce them in a large quantity at reasonable cost. Because many chemical and physical properties of nanomaterials are size- and shape-dependent, rational syntheses of nanomaterials to achieve desirable dimensionalities and morphologies are essential to exploit their utilities. In this Account, we show that the dislocation-driven growth mechanism, where screw dislocation defects provide self-perpetuating growth steps to enable the anisotropic growth of various nanomaterials at low supersaturation, can be a powerful and versatile synthetic method for a wide variety of nanomaterials. Despite significant progress in the last two decades, nanomaterial synthesis has often remained an "art", and except for a few well-studied model systems, the growth mechanisms of many anisotropic nanostructures remain poorly understood. We strive to go beyond the empirical science ("cook-and-look") and adopt a fundamental and mechanistic perspective to the anisotropic growth of nanomaterials by first understanding the kinetics of the crystal growth process. Since most functional nanomaterials are in single-crystal form, insights from the classical crystal growth theories are crucial. We pay attention to how screw dislocations impact the growth kinetics along different crystallographic directions and how the strain energy of defected crystals influences their equilibrium shapes. Furthermore, such inquiries are supported by detailed structural investigation to identify the evidence of dislocations. The dislocation-driven growth mechanism not only can unify the various explanations behind a wide variety of exotic nanoscale morphologies but also allows the rational design of catalyst-free solution-phase syntheses that could enable the scalable and low cost production of nanomaterials necessary for large scale applications, such as solar and thermoelectric energy conversions, energy storage, and nanocomposites. In this Account, we discuss the fundamental theories of the screw dislocation driven growth of various nanostructures including one-dimensional nanowires and nanotubes, two-dimensional nanoplates, and three-dimensional hierarchical tree-like nanostructures. We then introduce the transmission electron microscopy (TEM) techniques to structurally characterize the dislocation-driven nanomaterials for future searching and identifying purposes. We summarize the guidelines for rationally designing the dislocation-driven growth and discuss specific examples to illustrate how to implement the guidelines. By highlighting our recent discoveries in the last five years, we show that dislocation growth is a general and versatile mechanism that can be used to grow a variety of nanomaterials via distinct reaction chemistry and synthetic methods. These discoveries are complemented by selected examples of anisotropic crystal growth from other researchers. The fundamental investigation and development of dislocation-driven growth of nanomaterials will create a new dimension to the rational design and synthesis of increasingly complex nanomaterials.

Multigait soft robot.

This manuscript describes a unique class of locomotive robot: A soft robot, composed exclusively of soft materials (elastomeric polymers), which is inspired by animals (e.g., squid, starfish, worms) that do not have hard internal skeletons. Soft lithography was used to fabricate a pneumatically actuated robot capable of sophisticated locomotion (e.g., fluid movement of limbs and multiple gaits). This robot is quadrupedal; it uses no sensors, only five actuators, and a simple pneumatic valving system that operates at low pressures (< 10 psi). A combination of crawling and undulation gaits allowed this robot to navigate a difficult obstacle. This demonstration illustrates an advantage of soft robotics: They are systems in which simple types of actuation produce complex motion.

Interference Effects in Micro-Raman Spectroscopy Enable Mapping of Chemical Gradients on an Elastomer Surface.

Chemically modified elastomer surfaces are important to many applications, including microfluidics and soft sensors. Sensitive characterization of the interfacial chemistry of soft materials has been a persistent challenge, given their structural and chemical complexity. This article reports a method to probe local chemical states of elastomer surfaces that leverages the interference effects observed in micro-Raman spectroscopy. Unexpectedly, systematic variations of Raman scattering intensity were observed across a chemical wettability gradient grafted to the surface of a poly(dimethylsiloxane) (PDMS) film. Specifically, hydrophobic surface regions with a high graft density of long-chain hydrocarbon molecules showed suppressed Raman intensity. An optical interference model that accounts for molecular filling and swelling of an interfacial glassy layer during chemical modifications of the PDMS surface quantitatively reproduces experimental observations. This work establishes the spectroscopic signatures of interfacial chemical modifications on elastomer surfaces and enables a noncontact optical probe of local chemical states at the micro- and nanoscale compatible with the complex interfaces of soft materials.

Screw dislocation-driven growth of two-dimensional nanoplates.

We report the dislocation-driven growth of two-dimensional (2D) nanoplates. They are another type of dislocation-driven nanostructure and could find application in energy storage, catalysis, and nanoelectronics. We first focus on nanoplates of zinc hydroxy sulfate (3Zn(OH)(2)·ZnSO(4)·0.5H(2)O) synthesized from aqueous solutions. Both powder X-ray and electron diffraction confirm the zinc hydroxy sulfate (ZHS) crystal structure as well as their conversion to zinc oxide (ZnO). Scanning electron, atomic force, and transmission electron microscopy reveal the presence of screw dislocations in the ZHS nanoplates. We further demonstrate the generality of this mechanism through the growth of 2D nanoplates of α-Co(OH)(2), Ni(OH)(2), and gold that can also follow the dislocation-driven growth mechanism. Finally, we propose a unified scheme general to any crystalline material that explains the growth of nanoplates as well as different dislocation-driven nanomaterial morphologies previously observed through consideration of the relative crystal growth step velocities at the dislocation core versus the outer edges of the growth spiral under various supersaturations.

Chemical activation of commodity plastics for patterned electroless deposition of robust metallic films.

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 method is compatible with a diverse array of plastics and metals with properties applicable to flexible electronic circuits and electrochemical cells.

Rational solution growth of α-FeOOH nanowires driven by screw dislocations and their conversion to α-Fe2O3 nanowires.

We report the rational synthesis of α-FeOOH (goethite) nanowires following a dislocation-driven mechanism by utilizing a continuous-flow reactor and chemical equilibria to maintain constant low supersaturations. The existence of axial screw dislocations and the associated Eshelby twist in the nanowire product were confirmed using bright-/dark-field transmission electron microscopy imaging and twist contour analysis. The α-FeOOH nanowires can be readily converted into semiconducting single-crystal but porous α-Fe(2)O(3) (hematite) nanowires via topotactic transformation. Our results indicate that, with proper experimental design, many more useful materials can be grown in one-dimensional morphologies in aqueous solutions via the dislocation-driven mechanism.

Screw dislocation-driven epitaxial solution growth of ZnO nanowires seeded by dislocations in GaN substrates.

In the current examples of dislocation-driven nanowire growth, the screw dislocations that propagate one-dimensional growth originate from spontaneously formed highly defective "seed" crystals. Here we intentionally utilize screw dislocations from defect-rich gallium nitride (GaN) thin films to propagate dislocation-driven growth, demonstrating epitaxial growth of zinc oxide (ZnO) nanowires directly from aqueous solution. Atomic force microscopy confirms screw dislocations are present on the native GaN surface and ZnO nanowires grow directly from dislocation etch pits of heavily etched GaN surfaces. Furthermore, transmission electron microscopy confirms the existence of axial dislocations. Eshelby twist in the resulting ZnO nanowires was confirmed using bright-/dark-field imaging and twist contour analysis. These results further confirm the connection between dislocation source and nanowire growth. This may eventually lead to defect engineering strategies for rationally designed catalyst-free dislocation-driven nanowire growth for specific applications.

Dynamic manipulation of droplets using mechanically tunable microtextured chemical gradients.

Materials and strategies applicable to the dynamic transport of microdroplets are relevant to surface fluidics, self-cleaning materials, thermal management systems, and analytical devices. Techniques based on electrowetting, topographic micropatterns, and thermal/chemical gradients have advanced considerably, but dynamic microdroplet transport remains a challenge. This manuscript reports the fabrication of mechano-tunable, microtextured chemical gradients on elastomer films and their use in controlled microdroplet transport. Specifically, discreet mechanical deformations of these films enabled dynamic tuning of the microtextures and thus transport along surface-chemical gradients. The interplay between the driving force of the chemical gradient and the microtopography was characterized, facilitating accurate prediction of the conditions (droplet radius and roughness) which supported transport. In this work, the use of microtextured surface chemical gradients in mechano-adaptive materials with microdroplet manipulation functionality was highlighted.

Facile Production of Large-Area Cell Arrays Using Surface-Assembled Microdroplets.

Techniques that enable the spatial arrangement of living cells into defined patterns are broadly applicable to tissue engineering, drug screening, and cell-cell investigations. Achieving large-scale patterning with single-cell resolution while minimizing cell stress/damage is, however, technically challenging using existing methods. Here, a facile and highly scalable technique for the rational design of reconfigurable arrays of cells is reported. Specifically, microdroplets of cell suspensions are assembled using stretchable surface-chemical patterns which, following incubation, yield ordered arrays of cells. The microdroplets are generated using a microfluidic-based aerosol spray nozzle that enables control of the volume/size of the droplets delivered to the surface. Assembly of the cell-loaded microdroplets is achieved via mechanically induced coalescence using substrates with engineered surface-wettability patterns based on extracellular matrices. Robust cell proliferation inside the patterned areas is demonstrated using standard culture techniques. By combining the scalability of aerosol-based delivery and microdroplet surface assembly with user-defined chemical patterns of controlled functionality, the technique reported here provides an innovative methodology for the scalable generation of large-area cell arrays with flexible geometries and tunable resolution.

Assembly of nanocrystal arrays by block-copolymer-directed nucleation.

Growing in line: The surface chemistry of self-assembled nanostructured block copolymers is used to control the sites at which semiconducting metal sulfide nanocrystals nucleate and grow on a surface directly from aqueous solutions. This process is a new and general strategy for the bottom-up assembly of functional nanocrystalline materials for a variety of applications.

Mechanically Induced Hydrophobic Recovery of Poly(dimethylsiloxane) (PDMS) for the Generation of Surfaces with Patterned Wettability.

Silicone elastomers are used in a variety of "stretchable" technologies (e.g., wearable electronics and soft robotics) that require the elastomeric components to accommodate varying magnitudes of mechanical stress during operation; however, there is limited understanding of how mechanical stress influences the surface chemistry of these elastomeric components despite the potential importance of this property with regards to overall function. In this study, plasma-oxidized silicone (poly(dimethylsiloxane)) films were systematically subjected to various amounts of tensile stress and the resulting surface chemical changes were monitored using contact angle measurements, X-ray photoelectron spectroscopy, and gas chromatography-mass spectrometry. Understanding the influence of mechanical stress on these materials made possible the development of a facile method for the rapid, on-demand switching of surface wettability and the generation of surface wettability patterns and gradients. The use of mechanical stress to control surface wettability is broadly applicable to the fields of microfluidics, soft robotics, printing, and to the design of adaptable materials and sensors.