Christoph Weder

Stimuli-Responsive Polymer Nanocomposites Inspired by the Sea Cucumber Dermis

Sea cucumbers, like other echinoderms, have the ability to rapidly and reversibly alter the stiffness of their inner dermis. It has been proposed that the modulus of this tissue is controlled by regulating the interactions among collagen fibrils, which reinforce a low-modulus matrix. We report on a family of polymer nanocomposites, which mimic this architecture and display similar chemoresponsive mechanic adaptability. Materials based on a rubbery host polymer and rigid cellulose nanofibers exhibit a reversible reduction by a factor of 40 of the tensile modulus, for example, from 800 to 20 megapascals (MPa), upon exposure to a chemical regulator that mediates nanofiber interactions. Using a host polymer with a thermal transition in the regime of interest, we demonstrated even larger modulus changes (4200 to 1.6 MPa) upon exposure to emulated physiological conditions.

A versatile approach for the processing of polymer nanocomposites with self-assembled nanofibre templates

The incorporation of nanoparticles into polymers is a design approach that is used in many areas of materials science. The concept is attractive because it enables the creation of materials with new or improved properties by mixing multiple constituents and exploiting synergistic effects. One important technological thrust is the development of structural materials with improved mechanical and thermal characteristics. Equally intriguing is the possibility to design functional materials with unique optical or electronic properties, catalytic activity or selective permeation. The broad technological exploitation of polymer nanocomposites is, however, stifled by the lack of effective methods to control nanoparticle dispersion. We report a simple and versatile process for the formation of homogeneous polymer/nanofibre composites. The approach is based on the formation of a three-dimensional template of well-individualized nanofibres, which is filled with any polymer of choice. We demonstrate that this template approach is broadly applicable and allows for the fabrication of otherwise inaccessible nanocomposites of immiscible components.

Polymer Nanocomposites with Nanowhiskers Isolated from Microcrystalline Cellulose

The ability to produce polymer nanocomposites, which comprise a percolating, three-dimensional network of well-individualized nanofibers, is important to maximize the reinforcing effect of the nanofibers. While microcrystalline cellulose (MCC) has been previously shown to improve the mechanical properties of polymer composites, the formation of fibrous percolating networks within the nanocomposites has been stifled. Through the utilization of a template approach, nanocomposites based on an ethylene oxide/epichlorohydrin copolymer and nanowhiskers isolated from MCC were produced that display the maximum mechanical reinforcement predicted by the percolation model.

Shape memory polymers with built-in threshold temperature sensors

The design, fabrication and characterization of a new shape memory polymer (SMP) with built-in temperature sensing capabilities are reported. The material was prepared by incorporating a fluorescent, chromogenic oligo(p-phenylene vinylene) dye into a cross-linked poly(cyclooctene) (PCO) matrix via guest diffusion. The dye concentration was chosen to allow for self-assembly of the dye upon drying, resulting in the formation of excimers. Exposure of these phase-separated blends to temperatures above the melting point (Tm) of the PCO leads to dissolution of the dye molecules, and therefore causes a pronounced change of their absorption and fluorescence color. The optical changes are reversible; i.e., the aggregate absorption and emission are restored upon cooling below Tm. The color is dictated by the phase behavior and is independent of the mechanical state of the SMP. Thus the effect allows one to monitor reaching of the set/release temperature of the polymer.

Bio-inspired mechanically-adaptive nanocomposites derived from cotton cellulose whiskers

A new series of biomimetic, stimuli-responsive nanocomposites, which change their mechanical properties upon exposure to physiological conditions, was investigated. The materials were produced by introducing percolating networks of cellulose whiskers isolated from cotton into poly(vinyl acetate). Below the glass-transition temperature (Tg ∼ 63 °C), the tensile storage moduli (E′) of the dry nanocomposites increased two fold, from 2 GPa for the neat polymer to 4 GPa for a nanocomposite with 16.5% v/v whiskers. The relative reinforcement was more significant above Tg, where E′ was increased nearly 40 fold, from ∼1.2 MPa to ∼45 MPa. Upon exposure to emulated physiological conditions (immersion in artificial cerebrospinal fluid at 37 °C) all nanocomposites showed a pronounced decrease in E′, for example to 5 MPa for the 16.5% v/v whisker nanocomposites with only about 28% w/w swelling. This is a significant reduction in the amount of swelling required to decrease the E′, compared to earlier material versions based on cellulose whiskers with higher surface charge density; the decreased swelling may be a considerable advantage for the intended use of these materials as adaptive substrates for intracortical electrodes and other biomedical applications.

Mechanically adaptive intracortical implants improve the proximity of neuronal cell bodies

The hypothesis is that the mechanical mismatch between brain tissue and microelectrodes influences the inflammatory response. Our unique, mechanically adaptive polymer nanocomposite enabled this study within the cerebral cortex of rats. The initial tensile storage modulus of 5 GPa decreases to 12 MPa within 15 min under physiological conditions. The response to the nanocomposite was compared to surface-matched, stiffer implants of traditional wires (411 GPa) coated with the identical polymer substrate and implanted on the contralateral side. Both implants were tethered. Fluorescent immunohistochemistry labeling examined neurons, intermediate filaments, macrophages, microglia and proteoglycans. We demonstrate, for the first time, a system that decouples the mechanical and surface chemistry components of the neural response. The neuronal nuclei density within 100 µm of the device at four weeks post-implantation was greater for the compliant nanocomposite compared to the stiff wire. At eight weeks post-implantation, the neuronal nuclei density around the nanocomposite was maintained, but the density around the wire recovered to match that of the nanocomposite. The glial scar response to the compliant nanocomposite was less vigorous than it was to the stiffer wire. The results suggest that mechanically associated factors such as proteoglycans and intermediate filaments are important modulators of the response of the compliant nanocomposite.

Clay aerogel/cellulose whisker nanocomposites: a nanoscale wattle and daub

Aerogels based on either clay or cellulose nanofibers are representatives of an emerging class of structural materials with ultra-low density. Both types of aerogels are made from abundant raw materials and are formed through environmentally friendly freeze-drying processes. Due to the ultra-low-density layered superstructure that results from templating by the ice crystal morphology, the neat aerogels are unfortunately often rather fragile. The present study explores inorganic/organic hybrid aerogels that comprise montmorillonite and cellulose nanofibers isolated from tunicates. Dynamic mechanical testing revealed that, especially at low densities, these materials exhibit compressive strengths that are significantly higher than predicted by simple additive behavior of the properties of the individual components. At first glance, the data seem to suggest the formation of a nanoscale “wattle-and-daub”, in which the two components (mud-like clay and straw-like cellulose whiskers) complement each other. It appears that the main cause for this synergy is the enhanced formation of three dimensional network structures during the freeze-drying process.

Nanocomposites based on cellulose whiskers and (semi)conducting conjugated polymers

Two examples of a new class of functional nanocomposites derived from cellulose nanofibers (referred to as “whiskers”) and (semi)conducting π-conjugated polymers were prepared and studied. The conjugated polymers used were polyaniline (PANI) and a poly(p-phenylene ethynylene) (PPE) derivative with quaternary ammonium side chains. Cellulose whiskers with a typical diameter of around 20 nm, a length of around 1–2 µm, and anionic surface charges were combined with the positively charged π-conjugated polymers to form stable dispersions in polar solvents such as formic acid. Thin films were produced by solution-casting. Their composition was systematically varied to comprise between 65 and 99.4% w/w of the whiskers. Measurements of electrical conductivity, photoluminescence, and mechanical properties reveal that the nanocomposites synergistically combine the electronic characteristics of the conjugated polymers with the outstanding mechanical characteristics of the cellulose scaffold.

Polymeric Light-Emitting Diodes Based on Poly(p-phenylene ethynylene), Poly(triphenyldiamine), and Spiroquinoxaline

In this paper polymeric light-emitting diodes (LEDs) based on alkoxy-substituted poly(p-phenylene ethynylene) EHO-OPPE as emitter material in combination with poly(triphenyldiamine) as hole transport material are demonstrated. Different device configurations such as single-layer devices, two-layer devices, and blend devices were investigated. Device improvement and optimization were obtained through careful design of the device structure and composition. Furthermore, the influence of an additional electron transporting and hole blocking layer (ETHBL), spiroquinoxaline (spiro-qux), on top of the optimized blend device was investigated using a combinatorial method, which allows the preparation of a number of devices characterized by different layer thicknesses in one deposition step. The maximum brightness of the investigated devices increased from 4 cd/m2 for a device of pure EHO-OPPE to 260 cd/m2 in a device with 25 % EHO-OPPE + 75 % poly(N,N′-diphenylbenzidine diphenylether) (poly-TPD) as the emitting/hole-transporting layer and an additional electron-transport/hole-blocking spiro-qux layer of 48 nm thickness.

Mechanically-compliant intracortical implants reduce the neuroinflammatory response

OBJECTIVE The mechanisms underlying intracortical microelectrode encapsulation and failure are not well understood. A leading hypothesis implicates the role of the mechanical mismatch between rigid implant materials and the much softer brain tissue. Previous work has established the benefits of compliant materials on reducing early neuroinflammatory events. However, recent studies established late onset of a disease-like neurodegenerative state. APPROACH In this study, we implanted mechanically-adaptive materials, which are initially rigid but become compliant after implantation, to investigate the long-term chronic neuroinflammatory response to compliant intracortical microelectrodes. MAIN RESULTS Three days after implantation, during the acute healing phase of the response, the tissue response to the compliant implants was statistically similar to that of chemically matched stiff implants with much higher rigidity. However, at two, eight, and sixteen weeks post-implantation in the rat cortex, the compliant implants demonstrated a significantly reduced neuroinflammatory response when compared to stiff reference materials. Chronically implanted compliant materials also exhibited a more stable blood-brain barrier than the stiff reference materials. SIGNIFICANCE Overall, the data show strikingly that mechanically-compliant intracortical implants can reduce the neuroinflammatory response in comparison to stiffer systems.