Showing papers by "Christoph Weder published in 2015"

Progress towards biocompatible intracortical microelectrodes for neural interfacing applications

Abstract: To ensure long-term consistent neural recordings, next-generation intracortical microelectrodes are being developed with an increased emphasis on reducing the neuro-inflammatory response. The increased emphasis stems from the improved understanding of the multifaceted role that inflammation may play in disrupting both biologic and abiologic components of the overall neural interface circuit. To combat neuro-inflammation and improve recording quality, the field is actively progressing from traditional inorganic materials towards approaches that either minimizes the microelectrode footprint or that incorporate compliant materials, bioactive molecules, conducting polymers or nanomaterials. However, the immune-privileged cortical tissue introduces an added complexity compared to other biomedical applications that remains to be fully understood. This review provides a comprehensive reflection on the current understanding of the key failure modes that may impact intracortical microelectrode performance. In addition, a detailed overview of the current status of various materials-based approaches that have gained interest for neural interfacing applications is presented, and key challenges that remain to be overcome are discussed. Finally, we present our vision on the future directions of materials-based treatments to improve intracortical microelectrodes for neural interfacing.

Organogels for low-power light upconversion

Abstract: We herein report new organogels that permit efficient optical upconversion (UC) by triplet–triplet annihilation. The materials studied consist of a liquid organic phase, composed of a mixture of N,N-dimethylformamide and dimethyl sulfoxide in which the UC chromophore pair Pd(II) mesoporphyrin IX and 9,10-diphenylanthracene was dissolved, and a three-dimensional polymer network formed by covalently cross-linking poly(vinyl alcohol) with hexamethylene diisocyanate. The new gels are highly transparent, shape-persistent, and display efficient green-to-blue upconversion with UC quantum yields of >0.6 and 14% under ambient and oxygen-free conditions, respectively. The design approach presented here permits the fabrication of a hitherto unexplored class of materials with a unique combination of properties. The framework can easily be extended to other materials based on other solvents, polymer networks, and/or chromophore pairs.

Supramolecular Cross-Links in Poly(alkyl methacrylate) Copolymers and Their Impact on the Mechanical and Reversible Adhesive Properties

Abstract: Hydrogen-bonded, side-chain-functionalized supramolecular poly(alkyl methacrylate)s were investigated as light- and temperature-responsive reversible adhesives that are useful for bonding and debonding on demand applications. Here, 2-hydroxyethyl methacrylate (HEMA) was functionalized with 2-ureido-4[1H]pyrimidinone (UPy) via a hexamethylenediisocyanate (HMDI) linker, to create a monomer (UPy-HMDI-HEMA) that serves to form supramolecular cross-links by way of forming quadruple hydrogen bonded dimers. UPy-HMDI-HEMA was copolymerized with either hexyl methacrylate or butyl methacrylate to create copolymers comprising 2.5, 5, or 10 mol % of the cross-linker. The mechanical properties of all (co)polymers were investigated with stress–strain experiments and dynamic mechanical analysis. Furthermore, the adhesive properties were studied at temperatures between 20 and 60 °C by testing single lap joints formed with stainless steel substrates. It was found that increasing the concentration of the UPy-HMDI-HEMA cros...

Influence of Processing Conditions on Properties of Poly (Vinyl acetate)/Cellulose Nanocrystal Nanocomposites

Abstract: We investigated how different processing methods affect the morphology and mechanical properties of nanocomposites made from poly(vinyl acetate) (PVAc) and cellulose nanocrystals (CNCs). Homogeneously mixed reference PVAc/CNC nanocomposites of various compositions were first prepared by solution casting. These materials were post-processed by mixing in a roller blade mixer (RBM) or a twin-screw extruder (TSE) and subsequent compression molding. Transmission electron microscopy was used to elucidate the dispersion and size distribution of the CNCs and these data were correlated with the materials' mechanical properties. While RBM processed composites are virtually indistinguishable from the solution-cast reference materials, TSE processing led to mechanical degradation of the CNCs and resulted materials with inferior mechanical properties. Direct RBM mixing of PVAc and CNCs was also explored. This process afforded materials that are much stiffer than the neat matrix, but did not reach the levels of the reference series.

Melt processing of polyamide 12 and cellulose nanocrystals nanocomposites

Abstract: The fabrication of nanocomposites of polyamide 12 (PA12) and cellulose nanocrystals (CNCs) isolated from cotton and tunicates is reported. Through a comparative study that involved solution-cast (SC) and melt-processed materials, it was shown that PA12/CNC nanocomposites can be prepared in a process that appears to be readily scalable to an industrial level. The results demonstrate that CNCs isolated from the biomass by phosphoric acid hydrolysis display both a sufficiently high thermal stability to permit melt processing with PA12, and a high compatibility with this polymer to allow the formation of nanocomposites in which the CNCs are well dispersed. Thus, PA12/CNC nanocomposites prepared by melt-mixing the two components in a co-rotating roller blade mixer and subsequent compression molding display mechanical properties that are comparable to those of SC reference materials. Young's modulus and maximum stress could be doubled in comparison to the neat PA12 by introduction of 10% (CNCs from tunicates) or 15% w/w (CNCs from cotton) CNCs. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42752.

Influence of the nanofiber dimensions on the properties of nanocellulose/poly(vinyl alcohol) aerogels

Abstract: The investigation of aerogels made from cellulose nanofibers and poly(vinyl alcohol) (PVOH) as a polymeric binder is reported. Aerogels based on different nanocellulose types were studied to investigate the influence of the nanocellulose dimensions and their rigidity on the morphology and mechanical properties of the resulting aerogels. Thus, cellulose nanocrystals (CNCs) with low (10), medium (25), and high (80) aspect ratios, isolated from cotton, banana plants, and tunicates, respectively, microfibrillated cellulose (MFC) and microcrystalline cellulose (MCC) were dispersed in aqueous PVOH solutions and aerogels were prepared by freeze-drying. In addition to the cellulose type, the PVOH- and the CNC-concentration as well as the freeze-drying conditions were varied, and the materials were optionally cross-linked by an annealing step or the use of a chemical cross-linker. The data reveal that at low PVOH content, rigid, high-aspect ratio CNCs isolated from tunicates afford aerogels that show the least amount of shrinking upon freeze-drying and display the best mechanical properties. However, with increasing concentration of PVOH or upon introduc- tion of a chemical cross-linker the differences between materials made from different nanocellulose types decrease. V C 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41740.

Cellulose nanocrystal driven crystallization of poly(d,l-lactide) and improvement of the thermomechanical properties

Abstract: The technological exploitation of polylactide in fields requiring wide range of operating conditions is limited by the low crystallization rate of the polymer and therewith the low thermomechanical stability. Here we report the crystallization and consequent improvement of the thermomechanical properties of originally amorphous poly(d,l-lactide) (d : l ratio 11 : 89) loaded with cellulose nanocrystals (CNCs). Isothermal treatment of samples with different CNC contents and at various temperatures, showed up to 6 wt % crystalline phase formation, as confirmed by differential scanning calorimetry and X-ray diffraction measurements. Under a particular set of annealing conditions, CNCs promote the formation of a lamellar structure. This provides the system with higher order and cohesion which in combination with stress-transfer between CNCs, led to an increase of the storage modulus in the rubbery plateau up to 30 times (from 2.7 MPa up to 79 MPa), a rise of the melting temperature up to 50°C, and an improvement of the Young's modulus up to 40%. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41607.

Mechanochemistry in Polymers with Supramolecular Mechanophores.

Abstract: Mechanochemistry is a burgeoning field of materials science Inspired by nature, many scientists have looked at different ways to introduce weak bonds into polymeric materials to impart them with function and in particular mechano-responsiveness In the following sections, the incorporation of some of the weakest bonds, ie non-covalent bonds, into polymeric solids is being surveyed This review covers sequentially π–π interactions, H-bonding and metal-ligand coordination bonds and tries to highlight some of the advantages and limitations of such systems, while providing some key perspective of what may come next in this tantalizing field

Visualization of Polymer Deformation Using Microcapsules Filled with Charge-Transfer Complex Precursors.

Abstract: The visualization of polymer deformation using microcapsules filled with charge-transfer complex precursors is reported. Poly(urea-formaldehyde) microcapsules filled with toluene and either hexamethylbenzene as donor or chloranil as acceptor were prepared by in situ polymerization of an oil-in-water emulsion. The resulting microcapsules were incorporated in a poly(dimethylsiloxane) matrix by dispersing them in the monomer and subsequent curing. The microcapsules were shown to rupture when the polymer was placed under mechanical forces (i.e., tensile and compression) such that the donor and the acceptor are released into the polymeric matrix, where their combination to a charge-transfer complex results in a pronounced yellow-to-red color change. The factors influencing the extent of the color change (composition and mechanical strain) were investigated and a deformation threshold seems to be present. The new damage-sensing materials may be useful for a range of applications and the general concept applied ...

Shape-Memory Polyurethane Nanocomposites with Single Layer or Bilayer Oleic Acid-Coated Fe3O4 Nanoparticles

Abstract: The present study aimed at developing a shape-memory material, whose shape can be changed “on demand” upon application of an external magnetic field. Shape-memory nanocomposites were thus prepared from a commercially available thermoplastic polyurethane (PU) with a “switching temperature” of ≈40 °C and oleic acid (OA)-coated Fe3O4 nanoparticles (NPs). Two series of PU/Fe3O4 nanocomposite films based on the shape-memory PU and single layer or bilayer OA-coated Fe3O4 NPs were prepared by solution mixing followed by solvent casting. The thermal, mechanical, and shape-memory properties of the nanocomposite films were studied and compared.

Supramolecular Polymer Networks Made by Solvent-Free Copolymerization of a Liquid 2-Ureido-4[1H]-pyrimidinone Methacrylamide

Abstract: Supramolecular polymer networks based on polyacrylates with hydrogen bonding 2-ureido-4[1H]-pyrimidinone (UPy) side chains are of considerable interest due to the stimuli-responsive nature imparted by the reversible cross-links formed by dimerized UPy groups. Previously reported UPy-containing acrylic monomers are solid and show limited miscibility with comonomers, and this has stifled their (co)polymerization in bulk. We here report the synthesis of a liquid 2-ureido-4[1H]-pyrimidinone methacrylamide (UPy-OPG-MAA), which was made by connecting the UPy motif and methacrylamide (MAA) via an amine-terminated oligo(propylene glycol) (OPG) linker. The new monomer was miscible with conventional methacrylates. This permitted the photoinitiated free-radical bulk copolymerization with hexyl methacrylate (HMA) to afford a series of copolymers (poly(UPy-OPG-MAA-co-HMA)) in which the UPy-OPG-MAA content was varied between 0 and 20 mol %. The investigation of the mechanical properties of these copolymers by dynamic m...

Metal–organic frameworks: Framing upconversion materials

Abstract: Solid-state organic materials that convert low-power visible light into higher-energy radiation have been synthesized using metal–organic frameworks. This approach could be used to make polymers that increase the efficiency of photovoltaic devices.

Mechanically adaptive materials for intracortical implants

Abstract: Cortical microelectrodes allow electrical contacts with neural cells and show promise as electrical interfaces to the brain, which allow for the treatment of several neurological deficits. However, the functionality of current electrodes decreases over time, in part due to neuron degeneration and foreign body encapsulation. One hypothesis is that the mismatch of the mechanical properties of the electrode and the brain tissue is a significant contributor to these events. We recently developed a new approach to chemically responsive, mechanically adaptive polymer nanocomposites, which are initially highly rigid, but soften considerably upon exposure to physiological conditions and aqueous environments in general. Initial in-vivo experiments suggest that the materials promise to be a useful platform for the design of next-generation intracortical devices.

Physiologically responsive mechanically adaptive polymer optical fibers, production and methods of use

Abstract: Physiologically responsive mechanically adaptive optical fibers that are suitable for optical interfacing with living organisms. The optical fibers are particularly suited for applications in optogenetics. Dry, stiff fibers display a desirable tensile storage modulus and can be readily inserted into biological and in particular cortical tissue. Exposure to conditions encountered in vivo results in reduction, often a drastic reduction in modulus. When coupled with a suitable light source, the construction can be utilized to stimulate neurons in vivo. Methods for producing and utilizing the optical fibers and devices including the optical fibers are disclosed.

Charge-Transfer Complex Precursors

Abstract: The visualization of polymer deformation using microcapsules filled with charge-transfer complex precursors is reported. Poly(urea-formaldehyde) microcapsules filled with toluene and either hexamethylbenezene as donor or chloranil as acceptor were prepared by an in-situ polymerization of an oil-in-water emulsion. The resulting microcapsules were incorporated in a poly(dimethylsiloxane) matrix by dispersing them in the monomer and subsequent curing. The microcapsules were shown to rupture when the polymer was placed under mechanical forces (i.e., tensile and compression), such that the donor and the acceptor are released into the polymeric matrix, where their combination to a charge-transfer complex results in a pronounced yellow-to-red color change. The factors influencing the extent of the color change (composition and mechanical strain) were investigated and a deformation threshold seems to be present. The new damage-sensing materials may be useful for a range of applications and the general concept applied seems to be readily applicable to other host polymers.