Resilient yet entirely degradable gelatin-based biogels for soft robots and electronics.

Biodegradable and biocompatible elastic materials for soft robotics, tissue engineering or stretchable electronics with good mechanical properties, tunability, modifiability or healing properties drive technological advance, and yet they are not durable under ambient conditions and do not combine all the attributes in a single platform. We have developed a versatile gelatin-based biogel, which is highly resilient with outstanding elastic characteristics, yet degrades fully when disposed. It self-adheres, is rapidly healable and derived entirely from natural and food-safe constituents. We merge all the favourable attributes in one material that is easy to reproduce and scalable, and has a low-cost production under ambient conditions. This biogel is a step towards durable, life-like soft robotic and electronic systems that are sustainable and closely mimic their natural antetypes.

Functional adaptations in the tergite cuticle of the desert isopod Hemilepistus reaumuri (Milne-Edwards, 1840).

To survive in its extreme habitat, the cuticle of the burrowing desert isopod Hemilepistus reaumuri requires properties distinct from isopods living in moist or mesic habitats. In particular, the anterior tergites are exposed to high mechanical loads and temperatures when individuals guard the entrance of their burrow. We have, therefore, investigated the architecture, composition, calcite texture and local mechanical properties of the tergite cuticle, with particular emphasis on large anterior cuticle tubercles and differences between the anterior and posterior tergite. Unexpectedly, structure and thickness of the epicuticle resemble those in mesic isopod species. The anterior tergite has a thicker endocuticle and a higher local stiffness than the posterior tergite. Calcite distribution in the cuticle is unusual, because in addition to the exocuticle the endocuticle distally also contains calcite. The calcite consists of a distal layer of dense and highly co-oriented crystal-units, followed proximally by irregularly distributed and, with respect to each other, misoriented calcite crystallites. The calcite layer at the tip of the tubercle is thicker relative to the tubercle slopes, and its crystallites are more misoriented to each other. A steep decrease of local stiffness and hardness is observed within a distal region of the cuticle, likely caused by a successive increase in the ACC/calcite ratio rather than changes in the degree of mineralisation. Comparison of the results with other isopods reveals a much lower ACC/calcite ratio in H. reaumuri and a correlation between the degree of terrestriality of isopod species and the magnesium content of the cuticle.

Dynamic Supramolecular Ruthenium-Based Gels Responsive to Visible/NIR Light and Heat.

A simple supramolecular crosslinked gel is reported with a photosensitive ruthenium bipyridine complex functioning as a crosslinker and poly(4-vinylpyridine) (P4VP) as a macromolecular ligand. Irradiation of the organogels in H2 O/MeOH with visible and NIR light (in a multiphoton process) leads to cleavage of pyridine moieties from the ruthenium complex breaking the cross-links and causing degelation and hence solubilization of the P4VP chains. Real-time (RT) photorheology experiments of thin films showed a rapid degelation in several seconds, whereas larger bulk samples could also be photocleaved. Furthermore, the gels could be reformed or healed by simple heating of the system and restoration of the metal-ligand crosslinks. The relatively simple dynamic system with a high sensitivity towards light in the visible and NIR region make them interesting positive photoresists for nano/micropatterning applications, as was demonstrated by writing, erasing, and rewriting of the gels by single- and multiphoton lithography.

Grinding of nano-graphite inkjet inks for application in organic solar cells.

The production of printable graphene flakes is not easy to scale up when produced by ultrasonication and purified by centrifugation. In this work, natural graphite flakes were exfoliated by wet ball milling in water supported by the addition of sodium deoxycholate as a surfactant and the dispersion was formulated for inkjet printing. By subsequent dilution and filtration of the milling paste, more than 45 l of a stable dispersion of nano-graphite particles in one batch process was obtained. The dispersion was characterized by thermogravimetric analysis and UV-vis spectroscopy to determine concentration, and experiments to measure long-term stability were conducted. The nano-graphite particles were analyzed by optical microscopy, scanning electron microscopy and Raman spectroscopy, revealing 300-400 nm sized particles. The dispersion was formulated into an inkjet ink and tested as interfacial hole transport layer between the anode and the photo-active bulk-heterojunction layer of an organic solar cell with inverted structure. The nano-graphite flakes are inkjet printable and conductive and therefore show potential as a low-cost alternative to polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate.

Tailored disorder in calcite organization in tergite cuticle of the supralittoral isopod Tylos europaeus Arcangeli, 1938.

The crustacean cuticle forms skeletal elements consisting of chitin-protein fibrils reinforced by amorphous and crystalline calcium carbonate and phosphate minerals. The edges of skeletal elements are of particular interest. They are subject to repeated strain and stress because they form transitions to the arthrodial membranes connecting them. These allow for relative movements of skeletal elements. In this study, we investigate structure, chemical composition, mineral organization and local mechanical properties of the anterior and posterior edges of the tergite cuticle in the conglobating beach isopod Tylos europaeus and compare these with the protective dorsal region of the tergites. The distribution of mineral phases at the edges resembles that of dorsal regions of the tergites. At the transition with the unmineralized arthrodial membrane the calcite containing distal exocuticle is replaced by epicuticular material and the subjacent cuticular layers containing amorphous calcium carbonate become enriched with amorphous calcium phosphate. At the edges, the local elastic modulus and hardness values are significantly lower compared to dorsal regions of the tergite cuticle, for both, the calcite and the amorphous mineral containing layers. The calcite within the tergite cuticle is assembled in different texture patterns: (i) almost random co-orientation, (ii) almost single crystalline calcite, and (iii) a graded organization. Calcite organization and co-orientation strength is highly variable, not only on very few tens of micrometres, but also between regions with different skeletal functionality. Our results show that besides structure and composition, patterns of calcite organization contribute to the hierarchical architecture and functionality of biological composites.

Ultrastructure and mineral composition of the cornea cuticle in the compound eyes of a supralittoral and a marine isopod.

The cuticle of the cornea in Crustacea is an interesting example of a composite material compromising between two distinct functions. As part of the dioptric apparatus of the ommatidia within the complex eye it forms transparent micro-lenses that should as well maintain the mechanical stability of the head capsule. We analyzed the ultrastructure and composition of the isopod cornea cuticle of the terrestrial species Ligia oceanica and the marine Sphaeroma serratum. We used a variety of tissue preparation methods, electron microscopic techniques as well as electron microprobe analysis and Raman spectroscopic imaging. The results reveal various structural adaptations that likely increase light transmission. These are an increase in the thickness of the epicuticle, a reduction of the thickness of the outer layer of calcite, a spatial restriction of pore canals to interommatidial regions, and, for S. serratum only, an increase in calcite crystal size. In both species protein-chitin fibrils within the proximal exocuticle form a peculiar reticular structure that does not occur within the cuticle of the head capsule. In L. oceanica differential mineralization results in a spherically shaped interface between mineralized and unmineralized endocuticle, likely an adaptation to increase the refractive power of the cornea maintaining the mechanical stability of the cuticle between the ommatidia. The results show that the habitat and differences in the general structure of the animal's cuticle affect the way in which the cornea is adapted to its optical function.

Inorganic Bottlebrush and Comb Polymers as a Platform for Supersoft, Solvent-Free Elastomers.

Due to their unique rheological and mechanical properties, bottlebrush polymers are inimitable components of biological and synthetic systems such as cartilage and ultrasoft elastomers. However, while their rheological properties can be precisely controlled through their macromolecular structures, the current chemical spectrum available is limited to a handful of synthetic polymers with aliphatic carbon backbones. Herein we design and synthesize a series of inorganic bottlebrush polymers based on a unique combination of polydimethylsiloxane (PDMS) and polyphosphazene (PPz) chemistry. This non-carbon-based platform allows for simple variation of the significant architectural dimensions of bottlebrush-polymer-based elastomers. Grafting PDMS to PPz and vice versa also allows us to further exploit the unique properties of these polymers combined in a single material. These novel hybrid bottlebrush polymers were cured to give supersoft, solvent-free elastomers. We systematically studied the effect of architectural parameters and chemical functionality on their rheological properties. Besides forming supersoft elastomers, the energy dissipation characteristics of the elastomers were observed to be considerably higher than those for PDMS-based elastomers. Hence this work introduces a robust synthetic platform for solvent-free supersoft elastomers with potential applications as biomimetic damping materials.

The architecture of the joint head cuticle and its transition to the arthrodial membrane in the terrestrial crustacean Porcellio scaber.

The cuticle of terrestrial isopods is an interesting model for the study of structure-function relationships in biological composite materials. Its organic matrix has a hierarchically organised structure, and type and phase of the mineral compound can vary. The cuticle forms functionally diverse skeletal elements whose properties are adapted to their specific functions. In order to better understand the relation between structure, composition and function of isopod cuticle, we studied the structure and composition of the joint head that is part of the pereiopod's basis. It consists of a central region, whose shape fits well into the joint socket, and an edge region that is connected to the soft arthrodial membrane and protects the central region from mechanical load. The cuticle architecture of the joint head has local variations in structure and composition. In the central region the cuticle is similar to the previously published tergite cuticle. High concentrations of amorphous calcium phosphate are located in the endocuticle suggesting a coexistence with amorphous calcium carbonate. The edge region has an unexpected organisation characterised by thickening of the epi- and exocuticle and an unusual unidirectional orientation of chitin-protein fibrils within the endocuticle. The concentrations of phosphate are considerably higher than in the central region. The overall differentiation in the cuticular architecture of the edge in comparison to the central region reflects the adaptation to mechanical strains the cuticle has to sustain during contraction of extensor muscles, and to the structural and compositional transition from the edge to the connecting arthrodial membrane.

Assessing the nanoscale structure and mechanical properties of polymer monoliths used for chromatography.

Concerning polymeric monolithic materials utilized in separation science, the structural and mechanical characteristics from the nanoscopic to the macroscopic scale remain of great interest. Suitable analytical tools are urgently required to understand the polymer monolith's constituent structure, particularly in the case of nanoscale polymer properties that tend to develop gel porosity in contact with a mobile phase ultimately affecting the chromatographic performance. Herein described are our first findings from a characterization of commercially available analytical polymer monoliths based on styrene/divinylbenzene and methacrylate chemistries utilizing confocal Raman spectroscopy imaging and atomic force microscopy (AFM). Confocal Raman spectroscopy can be used to generate a three-dimensional representation of monoliths in both dry state and in contact with solvent. AFM force-indentation measurements on individual cross-sectioned globular features permit detailed assessment of mechanical properties of the stationary phase. This approach allowed so far unprecedented insight and identification of a heterogeneous cross-link density distribution of polymer material within individual globular features on a submicrometer scale.

Computational Modeling of Diffusion-Based Delamination for Active Implantable Medical Devices.

Delamination at heterogeneous material interfaces is one of the most prominent failure modes in active implantable medical devices (AIMDs). A well-known example of an AIMD is the cochlear implant (CI). In mechanical engineering, a multitude of testing procedures are known whose data can be used for detailed modeling with respect to digital twins. Detailed, complex models for digital twins are still lacking in bioengineering since body fluid infiltration occurs both into the polymer substrate and along the metal-polymer interfaces. For a newly developed test for an AIMD or CI composed of silicone rubber and metal wiring or electrodes, a mathematical model of these mechanisms is presented. It provides a better understanding of the failure mechanisms in such devices and their validation against real-life data. The implementation utilizes COMSOL Multiphysics®, consisting of a volume diffusion part and models for interface diffusion (and delamination). For a set of experimental data, the necessary diffusion coefficient could be derived. A subsequent comparison of experimental and modeling results showed a good qualitative and functional match. The delamination model follows a mechanical approach. The results of the interface diffusion model, which follows a substance transport-based approach, show a very good approximation to the results of previous experiments.

Improving Layer Adhesion of Co-Extruded Polymer Sheets by Inducing Interfacial Flow Instabilities.

Co-extrusion is commonly used to produce polymer multilayer products with different materials tailoring the property profiles. Adhesion between the individual layers is crucial to the overall performance of the final structure. Layer adhesion is determined by the compatibility of the polymers at the interface and their interaction forces, causing for example the formation of adhesive or chemical bonds or an interdiffusion layer. Additionally, the processing conditions, such as temperature, residence time, cooling rate, and interfacial shear stress, have a major influence on the interactions and hence resulting layer adhesion. Influences of temperature and residence time are already quite well studied, but influence of shear load on the formation of an adhesion layer is less explored and controversially discussed in existing literature. In this work, we investigated the influence of different processing conditions causing various shear loads on layer adhesion for a two-layer co-extruded polymer sheet using a polypropylene and polypropylene talc compound system. Therefore, we varied the flow rates and the flow geometry of the die. Under specific conditions interfacial flow instabilities are triggered that form micro layers in the transition regime between the two layers causing a major increase in layer adhesion. This structure was analyzed using confocal Raman microscopy. Making use of these interfacial flow instabilities in a controlled way enables completely new opportunities and potentials for multi-layer products.

Amorphous and crystalline calcium carbonate distribution in the tergite cuticle of moulting Porcellio scaber (Isopoda, Crustacea).

The main mineral components of the isopod cuticle consists of crystalline magnesium calcite and amorphous calcium carbonate. During moulting isopods moult first the posterior and then the anterior half of the body. In terrestrial species calcium carbonate is subject to resorption, storage and recycling in order to retain significant fractions of the mineral during the moulting cycle. We used synchrotron X-ray powder diffraction, elemental analysis and Raman spectroscopy to quantify the ACC/calcite ratio, the mineral phase distribution and the composition within the anterior and posterior tergite cuticle during eight different stages of the moulting cycle of Porcellio scaber. The results show that most of the amorphous calcium carbonate (ACC) is resorbed from the cuticle, whereas calcite remains in the old cuticle and is shed during moulting. During premoult resorption of ACC from the posterior cuticle is accompanied by an increase within the anterior tergites, and mineralization of the new posterior cuticle by resorption of mineral from the anterior cuticle. This suggests that one reason for using ACC in cuticle mineralization is to facilitate resorption and recycling of cuticular calcium carbonate. Furthermore we show that ACC precedes the formation of calcite in distal layers of the tergite cuticle.

Ultrastructure and mineral distribution in the tergite cuticle of the beach isopod Tylos europaeus Arcangeli, 1938.

The crustacean cuticle is a hierarchically organised material composed of an organic matrix and mineral. It is subdivided into skeletal elements whose physical properties are adapted to their function and the eco-physiological strains of the animal. Using a variety of ultrastructural and analytical techniques we studied the organisation of the tergite cuticle of the sand burrowing beach isopod Tylos europaeus. The surface of the tergites bear epicuticular scales, sensilla and micro-tubercles. A distal layer of the exocuticle is characterised by a low density of organic fibres and the presence of magnesium-calcite. Surprisingly, the mineral forms regions containing polyhedral structures alternating with smooth areas. Between sub-domains within the distal exocuticle calcite varies in its crystallographic orientation. Proximal layers of the exocuticle and the endocuticle are devoid of calcite and the mineral occurs in the form of amorphous calcium carbonate (ACC). Using thin sections of mineralised cuticle we describe for the first time that ACC forms tubes around single protein-chitin fibrils.

The dorsal tergite cuticle of Helleria brevicornis: Ultrastructure, mineral distribution, calcite microstructure and texture.

Among the terrestrial Crustacea, isopods have most successfully established themselves in a large variety of terrestrial habitats. As in most Crustacea, their cuticle consists of a hierarchically organised organic phase of chitin-protein fibrils, containing calcium carbonate and some calcium phosphate. In previous studies, we examined the tergite cuticle of Tylos europaeus, which lives on seashores and burrows into moist sand. In this study, we investigate the closely related species Helleria brevicornis, which is completely terrestrial and lives in leaf litter and humus and burrows into the soil. To get deeper insights in relation between the structure of the organic and mineral phase in species living in diverse habitats, we have investigated the structure, and the chemical and crystallographic properties of the tergite cuticle using various preparation techniques, and microscopic and analytical methods. The results reveal long and short epicuticular sensilla with brushed tips on the tergite surface that do not occur in T. europaeus. As in T. europaeus a distal exocuticle, which contains a low number of organic fibres, contains calcite while the subjacent layers of the exo- and endocuticle contain amorphous calcium carbonate. The distal exocuticle contains a polygonal pattern of mineral initiation sites that correspond to interprismatic septa described for decapod crabs. The shape and position of calcite units do not follow the polygonal pattern of the septa. The results indicate that the calcite units form by crystallisation from an amorphous phase that progresses from both margins of the septa to the centres of the polygons.

Predicting Corrosion Delamination Failure in Active Implantable Medical Devices: Analytical Model and Validation Strategy.

The ingress of body fluids or their constituents is one of the main causes of failure of active implantable medical devices (AIMDs). Progressive delamination takes its origin at the junctions where exposed electrodes and conductive pathways enter the implant interior. The description of this interface is considered challenging because electrochemically-diffusively coupled processes are involved. Furthermore, standard tests and specimens, with clearly defined 3-phase boundaries (body fluid-metal-polymer), are lacking. We focus on polymers as substrate and encapsulation and present a simple method to fabricate reliable test specimens with defined boundaries. By using silicone rubber as standard material in active implant encapsulation in combination with a metal surface, a corrosion-triggered delamination process was observed that can be universalised towards typical AIMD electrode materials. Copper was used instead of medical grade platinum since surface energies are comparable but corrosion occurs faster. The finding is that two processes are superimposed there: First, diffusion-limited chemical reactions at interfaces that undermine the layer adhesion. The second process is the influx of ions and body fluid components that leave the aqueous phase and migrate through the rubber to internal interfaces. The latter observation is new for active implants. Our mathematical description with a Stefan-model coupled to volume diffusion reproduces the experimental data in good agreement and lends itself to further generalisation.

Metal-Free Hydrogen-Bonded Polymers Mimic Noble Metal Electrocatalysts.

The most active and efficient catalysts for the electrochemical hydrogen evolution reaction (HER) rely on platinum, a fact that increases the cost of producing hydrogen and thereby limits the widespread adoption of this fuel. Here, a metal-free organic electrocatalyst that mimics the platinum surface by implementing a high work function and incorporating hydrogen-affine hydrogen bonds is introduced. These motifs, inspired from enzymology, are deployed here as selective reaction centres. It is shown that the keto-amine hydrogen-bond motif enhances the rate-determining step in proton reduction to molecular hydrogen. The keto-amine-functionalized polymers reported herein evolve hydrogen at an overpotential of 190 mV. They share certain key properties with platinum: a similar work function and excellent electrochemical stability and chemical robustness. These properties allow the demonstration of one week of continuous HER operation without notable degradation nor delamination from the carrier electrode. Scaled continuous-flow electrolysis is reported and 1 L net molecular hydrogen is produced within less than 9 h using 2.3 mg of polymer electrocatalyst.

Ultrastructure and mineral distribution in the tergal cuticle of the terrestrial isopod Titanethes albus. Adaptations to a karst cave biotope.

Composition and spatial distribution of organic and inorganic materials within the cuticle of isopods vary between species. These variations are related to the behaviour and habitat of the animal. The troglobiotic isopod Titanethes albus lives in the complete darkness of caves in the Slovenian Karst. This habitat provides constant temperature and saturated humidity throughout the year and inconsistent food supply. These conditions should have lead to functional adaptations of arthropod cuticles. However, studies on structure and composition of cave arthropod cuticles are rare and lacking for terrestrial isopods. We therefore analysed the tergite cuticle of T. albus using transmission and field-emission electron microscopy, confocal micro-Raman spectroscopic imaging, quantitative X-ray diffractometry, thermogravimetric analysis and atomic absorption spectroscopy. The ultrastructure of the epicuticle suggests a poor resistance against water loss. A weak interconnection between the organic and mineral phase within the endo- and exocuticle, a comparatively thin apical calcite layer, and almost lack of magnesium within the calcite crystal lattice suggest that the mechanical strength of the cuticle is low in the cave isopod. This may possibly be of advantage in maintaining high cuticle flexibility and reducing metabolic expenditures.

Spatial distribution of calcite and amorphous calcium carbonate in the cuticle of the terrestrial crustaceans Porcellio scaber and Armadillidium vulgare.

The crustacean cuticle is an interesting model to study the properties of mineralized bio-composites. The cuticle consists of an organic matrix composed of chitin-protein fibres associated with various amounts of crystalline and amorphous calcium carbonate. It is thought that in isopods the relative amounts of these mineral polymorphs depend on its function and the habitat of the animal. In addition to the composition, the distribution of the various components should affect the properties of the cuticle. However, the spatial distribution of calcium carbonate polymorphs within the crustacean cuticle is unknown. Therefore, we analyzed the mineralized cuticles of the terrestrial isopods Armadillidium vulgare and Porcellio scaber using scanning electron-microscopy, electron probe microanalysis and confocal mu-Raman spectroscopic imaging. We show for the first time that the mineral phases are arranged in distinct layers. Calcite is restricted to the outer layer of the cuticle that corresponds to the exocuticle. Amorphous calcium carbonate is located within the endocuticle that lies below the exocuticle. Within both layers mineral is arranged in rows of granules with diameters of about 20 nm. The results suggest functional implications of mineral distribution that accord to the moulting and escape behaviour of the animals.

Granular chitin in the epidermis of nudibranch molluscs.

Chitin is usually found in stiff extracellular coatings typified by the arthropod exoskeleton, and is not associated with the soft, flexible mollusc skin. Here, we show, however, that chitin in nudibranch gastropods (Opisthobranchia, Mollusca) occurs as intracellular granules that fill the epidermal cells of the skin and the epithelial cells of the stomach. In response to nematocysts fired by tentacles of prey Cnidaria, the epidermal cells of eolid nudibranchs (Aeolidacea) release masses of chitin granules, which then form aggregates with the nematocyst tubules, having the effect of insulating the animal from the deleterious action of the Cnidaria tentacles. Granular chitin, while protecting the animal, does not interfere with the suppleness and flexibility of the skin, in contrast to the stiffness of chitin armor. The specialized epidermis enables nudibranchs to live with and feed on Cnidaria.

Instant tough bonding of hydrogels for soft machines and electronics.

Introducing methods for instant tough bonding between hydrogels and antagonistic materials-from soft to hard-allows us to demonstrate elastic yet tough biomimetic devices and machines with a high level of complexity. Tough hydrogels strongly attach, within seconds, to plastics, elastomers, leather, bone, and metals, reaching unprecedented interfacial toughness exceeding 2000 J/m2. Healing of severed ionic hydrogel conductors becomes feasible and restores function instantly. Soft, transparent multilayered hybrids of elastomers and ionic hydrogels endure biaxial strain with more than 2000% increase in area, facilitating soft transducers, generators, and adaptive lenses. We demonstrate soft electronic devices, from stretchable batteries, self-powered compliant circuits, and autonomous electronic skin for triggered drug delivery. Our approach is applicable in rapid prototyping and in delicate environments inaccessible for extended curing and cross-linking.