Meredith N. Silberstein

Proton-Coupled Mechanochemical Transduction: A Mechanogenerated Acid

A novel mechanophore with acid-releasing capability is designed to produce a simple catalyst for chemical change in materials under mechanical stress. The mechanophore, based on a gem-dichlorocyclopropanated indene, is synthesized and used as a cross-linker in poly(methyl acrylate). Force-dependent rearrangement is demonstrated for cross-linked mechanophore samples loaded in compression, while the control shows no significant response. The availability of the released acid is confirmed by exposing a piece of insoluble compressed polymer to a pH indicator solution. The development of this new mechanophore is the first step toward force-induced remodeling of stressed polymeric materials utilizing acid-catalyzed cross-linking reactions.

Morphing Metal and Elastomer Bicontinuous Foams for Reversible Stiffness, Shape Memory, and Self-Healing Soft Machines.

A metal-elastomer-foam composite that varies in stiffness, that can change shape and store shape memory, that self-heals, and that welds into monolithic structures from smaller components is presented.

Modeling mechanophore activation within a crosslinked glassy matrix

Mechanically induced reactivity is a promising means for designing self-reporting materials. Mechanically sensitive chemical groups called mechanophores are covalently linked into polymers in order to trigger specific chemical reactions upon mechanical loading. These mechanophores can be linked either within the backbone or as crosslinks between backbone segments. Mechanophore response is sensitive to both the matrix properties and placement within the matrix, providing two avenues for material design. A model framework is developed to describe reactivity of mechanophores located as crosslinks in a glassy polymer matrix. Simulations are conducted at the molecular and macromolecular scales in order to develop macroscale constitutive relations. The model is developed specifically for the case of spiropyran (SP) in lightly crosslinked polymethylmethacrylate (PMMA). This optically trackable mechanophore (fluorescent when activated) allows the model to be assessed in terms of observed experimental behavior. Th...

Designing a retrievable and scalable cell encapsulation device for potential treatment of type 1 diabetes

Significance Cell encapsulation holds great potential as a better treatment for type 1 diabetes. An encapsulation system that is scalable to a clinically relevant capacity and can be retrieved or replaced whenever needed is highly desirable for clinical applications. Here we report a cell encapsulation device that is readily scalable and conveniently retrievable through a minimally invasive laparoscopic procedure. We demonstrated its mechanical robustness and facile mass transfer as well as its durable function in diabetic mice. We further showed, as a proof of concept, its scalability and retrievability in dogs. We believe this encapsulation device may contribute to a cellular therapy for type 1 diabetes and potentially other endocrine disorders and hormone-deficient diseases. Cell encapsulation has been shown to hold promise for effective, long-term treatment of type 1 diabetes (T1D). However, challenges remain for its clinical applications. For example, there is an unmet need for an encapsulation system that is capable of delivering sufficient cell mass while still allowing convenient retrieval or replacement. Here, we report a simple cell encapsulation design that is readily scalable and conveniently retrievable. The key to this design was to engineer a highly wettable, Ca2+-releasing nanoporous polymer thread that promoted uniform in situ cross-linking and strong adhesion of a thin layer of alginate hydrogel around the thread. The device provided immunoprotection of rat islets in immunocompetent C57BL/6 mice in a short-term (1-mo) study, similar to neat alginate fibers. However, the mechanical property of the device, critical for handling and retrieval, was much more robust than the neat alginate fibers due to the reinforcement of the central thread. It also had facile mass transfer due to the short diffusion distance. We demonstrated the therapeutic potential of the device through the correction of chemically induced diabetes in C57BL/6 mice using rat islets for 3 mo as well as in immunodeficient SCID-Beige mice using human islets for 4 mo. We further showed, as a proof of concept, the scalability and retrievability in dogs. After 1 mo of implantation in dogs, the device could be rapidly retrieved through a minimally invasive laparoscopic procedure. This encapsulation device may contribute to a cellular therapy for T1D because of its retrievability and scale-up potential.

Mechanochemical functionalization of disulfide linked hydrogels

Poly(ethylene glycol) hydrogels with disulfide linkages are functionalized through applied force. Compression or tension induces bond rupture at the relatively weak disulfide linkages, which will subsequently react through Michael-type addition with an acceptor molecule within the gel. We demonstrate the utility of this approach by patterning cell adhesion proteins through compression of a lithographically structured stamp, where cells predominately adhere to the compressed regions.

Multiscale Modeling of Mechanoresponsive Polymers

Mechanically-induced reactivity is a promising means for designing self sensing and autonomous materials. Mechanically sensitive chemical groups called mechanophores can be covalently linked into polymers in order to trigger specific chemical reactions upon mechanical loading. Here, glassy PMMA and rubbery PMA each linked with mechanochromic molecules, are used as representative polymer architectures for building both understanding and quantitative models of mechanophore activation in bulk polymers.

Mechanics of Persulfonated Polytetrafluorethylene Proton Exchange Membranes

Fuel cells enable direct chemical to electrical conversion of fuel to electricity, providing an efficient and clean process. Proton Exchange Membrane Fuel Cells (PEMFC), in which protons from hydrogen or methane cross a membrane to react with oxygen producing electricity, are the preferred transportable fuel cell. Nafion, a phase separated perfluorosulfonated ionomer, is the current benchmark membrane but still exhibits limited lifetime due to stress encountered during constrained cyclic hygro-thermal loading. In previous work [1] the viscoplastic nature of Nafion under uniaxial extension was experimentally characterized. The experimental results were used to develop a three-dimensional constitutive model which was then implemented within a finite element analysis software package. These results will be briefly reviewed. Here, the model is used to simulate the rate dependence of the stress and strain evolution in Nafion under the loading typically encountered during fuel cell operation. This loading consists of hygro-thermal cycling within partially constrained boundary conditions defined by other components of the fuel cell. This information could be used to either change the startup/shutdown procedure for a fuel cell or to guide the procedures used for accelerated membrane lifetime testing.

Microstructural Evolution of Nafion During Uniaxial Deformation Monitored by X-ray Scattering

Fuel cells enable direct chemical to electrical conversion of fuel to electricity, providing an efficient and clean process. Proton Exchange Membrane Fuel Cells (PEMFC), in which protons from hydrogen or methane cross a membrane to react with oxygen producing electricity, are the preferred transportable fuel cell. Nafion is the membrane of choice for Proton Exchange Membrane Fuel Cells (PEMFC) because its unique microstructure allows rapid transport of protons in a hydrated environment while maintaining mechanical integrity. The teflon-like backbone is hydrophobic while the sulfonated side chains are hydrophilic. In the presence of water molecules this causes the side chains to aggregate into clusters which contain most of the water while the backbone remains relatively dry. Extensive studies have been conducted in order to deduce the size and shape of the microstructural features in order to gain insight into its superior electrochemical and mechanical characteristics, and how they can be further improved (e.g. [1-7]). The shape of these regions (sphere, cylinders, ribbons etc) and their evolution with deformation is still a matter of debate. Here the microstructural evolution is monitored during uniaxial tensile testing via small and wide angle x-ray scattering. Two dimensional scattering profiles are recorded along with stress and strain as a function of time for monotonic, cyclic, and stress relaxation loading histories. These profiles are then reduced to amplitude, location, and orientation for each of the major structural peaks. The scattering data are interpreted in conjunction with existing literature in order to understand the rate and deformation dependent microstructural evolution and its relation to the rate dependent elastic-plastic stress-strain behavior exhibited by Nafion.