Structural and rheological aging in model attraction-driven glasses by Rheo-SANS.

Aging in a model colloidal suspension comprised of particles with a thermoreversible attraction is studied using Rheo-SANS techniques in the attractive-driven glass state. Multiple thermal pathways lead to a common rheological and microstructural aging trajectory, as was observed previously for a thermoreversible gel. SANS measurements of the colloidal glass microstructure as a function of temperature and time during various quench protocols are quantitatively characterized in terms of an effective interaction strength that becomes an order parameter defining the microstructural state of the glass. Using previously validated concepts of a fictive temperature, a semi-empirical, quantitative relationship similar to an Avrami relationship is established between the mechanical aging (elastic modulus) and microstructural aging (order parameter) that is independent of thermal history for the thermal profiles studied herein at long times. Furthermore, shear rejuvenation is studied, and while shear may only partially reduce the degree of structure in the glass, aging upon flow cessation is found to follow a common trajectory when viewed in terms of the microstructural order parameter.

Plasticity variable collagen-PEG interpenetrating networks modulate cell spreading.

The extracellular matrix protein collagen I has been used extensively in the field of biomaterials due to its inherent biocompatibility and unique viscoelastic and mechanical properties. Collagen I self-assembly into fibers and networks is environmentally sensitive to gelation conditions such as temperature, resulting in gels with distinct network architectures and mechanical properties. Despite this, collagen gels are not suitable for many applications given their relatively low storage modulus. We have prepared collagen-poly(ethylene glycol) [PEG] interpenetrating network (IPN) hydrogels to reinforce the collagen network, which also induces changes to network plasticity, a recent focus of study in cell-matrix interactions. Here, we prepare collagen/PEG IPNs, varying collagen concentration and collagen gelation temperature to assess changes in microarchitecture and mechanical properties of these networks. By tuning these parameters, IPNs with a range of stiffness, plasticity and pore size are obtained. Cell studies suggest that matrix plasticity is a key determinant of cell behavior, including cell elongation, on these gels. This work presents a natural/synthetic biocompatible matrix that retains the unique structural properties of collagen networks with increased storage modulus and tunable plasticity. The described IPN materials will be of use for applications in which control of cell spreading is desirable, as only minimal changes in sample preparation lead to changes in cell spreading and circularity. Additionally, this study contributes to our understanding of the connection between collagen self-assembly conditions and matrix structural and mechanical properties and presents them as useful tools for the design of other collagen based biomaterials. STATEMENT OF SIGNIFICANCE: We developed a collagen-poly(ethylene glycol) interpenetrating network (IPN) platform that retains native collagen architecture and biocompatibility but provides higher stiffness and tunable plasticity. With minor changes in collagen gelation temperature or concentration, IPN gels with a range of plasticity, storage modulus, and pore size can be obtained. The tunable plasticity of the gels is shown to modulate cell spreading, with a greater proportion of elongated cells on the most plastic of IPNs, supporting the assertion that matrix plasticity is a key determinant of cell spreading. The material can be of use for situations where control of cell spreading is desired with minimal intervention, and the findings herein may be used to develop similar collagen based IPN platforms.

Itaconic Acid as a Comonomer in Betulin-Based Thermosets via Sequential and Bulk Preparation.

The inherent chemical functionalities of biobased monomers enable the production of renewably sourced polymers that further advance sustainable manufacturing. Itaconic acid (IA) is a nontoxic, commercially produced biobased monomer that can undergo both UV and thermal curing. Betulin is a biocompatible, structurally complex diol derived from birch tree bark that has been recently studied for materials with diverse applications. Here, betulin, IA, and biobased linear diacids, 1,12-dodecanedioic acid (C12) and 1,18-octadecanedioic acid (C18), were used to prepare thermosets using sequential and bulk curing methods. Thermoplastic polyester precursors were synthesized and formulated into polyester-methacrylate (PM) resins to produce sequential UV-curable thermosets. Bulk-cured polyester thermosets were prepared using a one-pot, solventless melt polycondensation using glycerol as a cross-linker. The structure-property relationships of the thermoplastic polyester precursors, sequentially prepared PM thermosets, and bulk-cured polyester thermosets were evaluated with varying IA content. Both types of thermosets exhibited higher storage moduli, Tgs, and thermal stabilities with greater IA comonomer content. These results demonstrate the viability of using IA as a comonomer to produce betulin-based thermosets each with tunable properties, expanding the scope of their applications and use in polymeric materials.

Optimizing the alignment of thermoresponsive poly(N-isopropyl acrylamide) electrospun nanofibers for tissue engineering applications: A factorial design of experiments approach.

Thermoresponsive polymers, such as poly(N-isopropyl acrylamide) (PNIPAM), have been identified and used as cell culture substrates, taking advantage of the polymer's lower critical solution temperature (LCST) to mechanically harvest cells. This technology bypasses the use of biochemical enzymes that cleave important cell-cell and cell-matrix interactions. In this study, the process of electrospinning is used to fabricate and characterize aligned PNIPAM nanofiber scaffolds that are biocompatible and thermoresponsive. Nanofiber scaffolds produced by electrospinning possess a 3D architecture that mimics native extracellular matrix, providing physical and chemical cues to drive cell function and phenotype. We present a factorial design of experiments (DOE) approach to systematically determine the effects of different electrospinning process parameters on PNIPAM nanofiber diameter and alignment. Results show that high molecular weight PNIPAM can be successfully electrospun into both random and uniaxially aligned nanofiber mats with similar fiber diameters by simply altering the speed of the rotating mandrel collector from 10,000 to 33,000 RPM. PNIPAM nanofibers were crosslinked with OpePOSS, which was verified using FTIR. The mechanical properties of the scaffolds were characterized using dynamic mechanical analysis, revealing an order of magnitude difference in storage modulus (MPa) between cured and uncured samples. In summary, cross-linked PNIPAM nanofiber scaffolds were determined to be stable in aqueous culture, biocompatible, and thermoresponsive, enabling their use in diverse cell culture applications.

Dynamic Bonds in Covalently Crosslinked Polymer Networks for Photoactivated Strengthening and Healing.

A photoactivated-strengthening polymer network is reported. This approach incorporates dynamic bonds into the network architecture, which enables a secondary polymerization triggered by UV light. Three attributes of this material are demonstrated, including: i) there is simultaneous photoinduced strengthening and healing after the material is severed, ii) bulk property changes are spatially confined via photopatterning, and iii) there is permanent shape change post-irradiation.