The rates of duplex formation for two octamers of DNA (5' d-CACGGCTC/5' d-GAGCCGTG and 5' d-CACAGCAC/5' d-GTGCTGTG), the homologous RNA, and both sets of hybrids in 1 M NaCl buffer have been measured using stopped-flow spectroscopy. In addition, the thermodynamic parameters, ΔH° and ΔS°, have been determined for the same sequences under the same buffer conditions using optical melting techniques. These data reveal a linear free energy relationship between the free energy of activation for denaturation and the change in free energy for formation of the duplexes. This relationship indicates that these duplex formation reactions occur through a common unstructured transition state that is more similar to the single strands in solution than to the ensuing duplex. In addition, these data confirm that the greater stability of RNA duplexes relative to that of homologous DNA and hybrid duplexes is controlled by the denaturation rate and not the duplex formation rate.

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3D printing is a rapidly growing field that requires the development of yield-stress fluids that can be used in post-printing transformation processes. There are a limited number of yield-stress fluids currently available with the desired rheological properties for building structures with small filaments (≤ l00 μm) with high shape-retention. We present a printingcentric approach for 3D printing particle-free silicone oil-in-water emulsions with a polymer additive, poly (ethylene oxide). This particular material structure and formulation is used to build 3D structure and to pattern at filament diameters below that of any other known material in this class. Increasing the molecular weight of poly (ethylene oxide) drastically increases the extensibility of the material without significantly affecting shear flow properties (shear yield stress and linear viscoelastic moduli). Higher extensibility of the emulsion correlates to the ability of filaments to span relatively large gaps (greater than 6 mm) when extruded at large tip diameters (330 μm) and the ability to extrude filaments at high print rates (20 mm/s). 3D printed structures with these extensible particle-free emulsions undergo post-printing transformation, which converts them into elastomers. These elastomers can buckle and recover from extreme compressive strain with no permanent deformation, a characteristic not native to the emulsion.

Materials chemistries for hydrogel scaffolds that are capable of programming temporal (4D) attributes of cellular decision‐making in supported 3D microcultures are described. The scaffolds are fabricated using direct‐ink writing (DIW)—a 3D‐printing technique using extrusion to pattern scaffolds at biologically relevant diameters (≤ 100 µm). Herein, DIW is exploited to variously incorporate a rheological nanoclay, Laponite XLG (LAP), into 2‐hydroxyethyl methacrylate (HEMA)‐based hydrogels—printing the LAP–HEMA (LH) composites as functional modifiers within otherwise unmodified 2D and 3D HEMA microstructures. The nanoclay‐modified domains, when tested as thin films, require no activating (e.g., protein) treatments to promote robust growth compliances that direct the spatial attachment of fibroblast (3T3) and preosteoblast (E1) cells, fostering for the latter a capacity to direct long‐term osteodifferentiation. Cell‐to‐gel interfacial morphologies and cellular motility are analyzed with spatial light interference microscopy (SLIM). Through combination of HEMA and LH gels, high‐resolution DIW of a nanocomposite ink (UniH) that translates organizationally dynamic attributes seen with 2D gels into dentition‐mimetic 3D scaffolds is demonstrated. These analyses confirm that the underlying materials chemistry and geometry of hydrogel nanocomposites are capable of directing cellular attachment and temporal development within 3D microcultures—a useful material system for the 4D patterning of hydrogel scaffolds.

Direct-ink writing (DIW), a rapidly growing and advancing form of additive manufacturing, provides capacities for on-demand tailoring of materials to meet specific requirements for final designs. The penultimate challenge faced with the increasing demand of customization is to extend beyond modification of shape to create 4D structures, dynamic 3D structures that can respond to stimuli in the local environment. Patterning material gradients is foundational for assembly of 4D structures, however, there remains a general need for useful materials chemistries to generate gray scale gradients via DIW. Here, presented is a simple materials assembly paradigm using DIW to pattern ionotropic gradients in hydrogels. Using structures that architecturally mimic seajelly organisms, the capabilities of spatial patterning are highlighted as exemplified by selectively programming the valency of the ion-binding agents. Spatial gradients, when combined with geometry, allow for programming the flexibility and movement of iron oxide nanoparticleloaded ionotropic hydrogels to generate 4D-printed structures that actuate in the presence of local magnetic fields. This work highlights approaches to 4D design complexity that exploits 3D-printed gray-scale/gradient mechanics.