Valerie Sheares Ashby

Dynamic Topographical Control of Mesenchymal Stem Cells by Culture on Responsive Poly(ϵ‐caprolactone) Surfaces

There is clear, emerging evidence in the literature supporting the influence of surface topography on various cell phenotypes.[1–5] Recent advancements in mechanobiology have relied heavily on synthetic extracellular matrix (ECM) mimics to investigate how cellular phenomena are dependent upon surface geometry. Concurrent developments in micro/nano-fabrication techniques have enabled the construction of well-defined surface arrays which aim to emulate the extracellular microenvironment.[6] Numerous patterns of different sizes and shapes including grooves, posts, and pits have been used to study the in vitro response of various cell types such as: fibroblasts, osteoblasts, epithelial cells, neuronal cells, and more recently stem cells.[7–14]

Switchable Micropatterned Surface Topographies Mediated by Reversible Shape Memory

Reversibly switching topography on micrometer length scales greatly expands the functionality of stimuli-responsive substrates. Here we report the first usage of reversible shape memory for the actuation of two-way transitions between microscopically patterned substrates, resulting in corresponding modulations of the wetting properties. Reversible switching of the surface topography is achieved through partial melting and recrystallization of a semi-crystalline polyester embossed with microscopic features. This behavior is monitored with atomic force microscopy (AFM) and contact angle measurements. We demonstrate that the magnitude of the contact angle variations depends on the embossment pattern.

It Is the Outside That Counts: Chemical and Physical Control of Dynamic Surfaces

Materials capable of dynamically controlling surface chemistry and topography are highly desirable. We have designed a system that is uniquely able to remotely control the presented functionality and geometry at a given time by using a functionalizable shape memory material. This was accomplished by incorporating controlled amounts of an azide-containing monomer into a shape memory polymeric material. These materials are capable of physically changing surface geometry over a broad range of length scales from >1 mm to 100 nm. Using copper-assisted click chemistry, they can be functionalized with a variety of molecules to yield different surfaces. Combining these features gave materials that can change both the presented geometry and functionality at tunable transition temperatures.

Synthesis, crystal structures, and electronic properties of nonlinear fused thienoacene semiconductors.

Two fused thienoacene compounds with two-dimensional ring connectivity were synthesized, and their semiconducting properties were characterized. Both compounds have a crystal structure comprised of herringbone arrays of tight π-π stacks. Strong π-π interactions lead to self-assembly into well-defined crystalline thin films from the vapor phase for both compounds. Field effect transistors were fabricated, affording identical hole mobilities of 3.0 × 10(-3) cm(2)/(V s) and I(on/off) > 10(5).

Dynamic Optical Gratings Accessed by Reversible Shape Memory

Shape memory polymers (SMPs) have been shown to accurately replicate photonic structures that produce tunable optical responses, but in practice, these responses are limited by the irreversibility of conventional shape memory processes. Here, we report the intensity modulation of a diffraction grating utilizing two-way reversible shape changes. Reversible shifting of the grating height was accomplished through partial melting and recrystallization of semicrystalline poly(octylene adipate). The concurrent variations of the grating shape and diffraction intensity were monitored via atomic force microscopy and first order diffraction measurements, respectively. A maximum reversibility of the diffraction intensity of 36% was repeatable over multiple cycles. To that end, the reversible shape memory process is shown to broaden the functionality of SMP-based optical devices.

Perfluoroalkyl-substitution versus electron-deficient building blocks in design of oligothiophene semiconductors

A series of oligothiophenes containing either electron withdrawing perfluorohexyl substituents or an electron deficient benzothiadiazole core, or both, were synthesized and their properties were evaluated by optical spectroscopy, cyclic voltammetry and DFT calculations. The charge transport properties of these compounds were studied in field effect transistors, revealing how the majority charge carrier is affected by the chemical substitution. Incorporation of benzothiadiazole significantly lowers the LUMO energy of the oligomers, but does not affect the HOMO. On the other hand, perfluoroalkyl groups lowered both the HOMO and LUMO, and were essential for achieving electron conductivity in this series of compounds.

Grafting poly(OEGMA) brushes from a shape memory elastomer and subsequent wrinkling behavior

An azide-functionalized shape memory elastomer, poly(octylene diazoadipate-co-octylene adipate), has been grafted with poly(oligoethylene glycol) methacrylate (poly(OEGMA)) brushes via aqueous ARGET (activators regenerated by electron transfer) ATRP. Sequential swelling of the substrate followed by a grafting-from reaction yielded an incompressible brush layer on the shape-memory substrate. Upon heating the substrate above the Tm to return to the primary shape, uniaxial wrinkles perpendicular to the direction of strain with sizes of 27-33 μm appear in addition to micrometer-sized features formed on the temporary shape after grafting. Swelling equilibration time (t1) and grafting reaction time (t2) were varied to control wrinkle formation and size. In this manner, we were able to create unique, anisotropic hierarchical surface structures with different length scales and patterns.

Shape memory particles capable of controlled geometric and chemical asymmetry made from aliphatic polyesters.

A novel method for producing monodisperse micro- and nanosized shape memory particles from various shape memory polymers (SMPs) is reported. This method uses a polydimethylsiloxane mold to uniformly deform particles from complex shapes to other well-defined shapes, harvest them without aggressive solvents or heat, and then return them to their original shapes upon heating above a preselected trigger temperature. By manipulating the material properties of both the mold and SMP, monodisperse asymmetric particles are easily achieved. This method is demonstrated with traditional SMPs and polymers with varying degrees of reactive functionality, crystallinity, and transition temperature. This additional reactivity and the robustness of this system allow easy tailoring of the surface with click chemistry to achieve chemical asymmetry.