- Award ID(s):
- NSF-PAR ID:
- Date Published:
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- Trends in Biomaterials and Artificial Organs
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Synopsis The adhesive toe pads of tree frogs have inspired the design of various so-called ‘smooth’ synthetic adhesives for wet environments. However, these adhesives do not reach the attachment performance of their biological models in terms of contact formation, maintenance of attachment, and detachment. In tree frogs, attachment is facilitated by an interconnected ensemble of superficial and internal morphological components, which together form a functional unit. To help bridging the gap between biological and bioinspired adhesives, in this review, we (1) provide an overview of the functional components of tree frog toe pads, (2) investigate which of these components (and attachment mechanisms implemented therein) have already been transferred into synthetic adhesives, and (3) highlight functional analogies between existing synthetic adhesives and tree frogs regarding the fundamental mechanisms of attachment. We found that most existing tree-frog-inspired adhesives mimic the micropatterned surface of the ventral epidermis of frog pads. Geometrical and material properties differ between these synthetic adhesives and their biological model, which indicates similarity in appearance rather than function. Important internal functional components such as fiber-reinforcement and muscle fibers for attachment control have not been considered in the design of tree-frog-inspired adhesives. Experimental work on tree-frog-inspired adhesives suggests that the micropatterning of adhesives with low-aspect-ratio pillars enables crack arresting and the drainage of interstitial liquids, which both facilitate the generation of van der Waals forces. Our analysis of experimental work on tree-frog-inspired adhesives indicates that interstitial liquids such as the mucus secreted by tree frogs play a role in detachment. Based on these findings, we provide suggestions for the future design of biomimetic adhesives. Specifically, we propose to implement internal fiber-reinforcements inspired by the fibrous structures in frog pads to create mechanically reinforced soft adhesives for high-load applications. Contractile components may stimulate the design of actuated synthetic adhesives with fine-tunable control of attachment strength. An integrative approach is needed for the design of tree-frog-inspired adhesives that are functionally analogous with their biological paradigm.more » « less
Bioadhesives such as tissue adhesives, hemostatic agents, and tissue sealants have potential advantages over sutures and staples for wound closure, hemostasis, and integration of implantable devices onto wet tissues. However, existing bioadhesives display several limitations including slow adhesion formation, weak bonding, low biocompatibility, poor mechanical match with tissues, and/or lack of triggerable benign detachment. Here, we report a bioadhesive that can form instant tough adhesion on various wet dynamic tissues and can be benignly detached from the adhered tissues on demand with a biocompatible triggering solution. The adhesion of the bioadhesive relies on the removal of interfacial water from the tissue surface, followed by physical and covalent cross-linking with the tissue surface. The triggerable detachment of the bioadhesive results from the cleavage of bioadhesive’s cross-links with the tissue surface by the triggering solution. After it is adhered to wet tissues, the bioadhesive becomes a tough hydrogel with mechanical compliance and stretchability comparable with those of soft tissues. We validate in vivo biocompatibility of the bioadhesive and the triggering solution in a rat model and demonstrate potential applications of the bioadhesive with triggerable benign detachment in ex vivo porcine models.
Achieving strong adhesion between wet materials (i.e., tissues and hydrogels) is challenging. Existing adhesives are weak, toxic, incompatible with wet and soft surfaces, or restricted to specific functional groups from the wet materials. The approach reported here uses biocompatible polymer chains to achieve strong adhesion and retain softness, but requires no functional groups from the wet materials. In response to a trigger, the polymer chains form a network, in topological entanglement with the two polymer networks of the wet materials, stitching them together like a suture at the molecular scale. To illustrate topological adhesion, pH is used as a trigger. The stitching polymers are soluble in water in one pH range but form a polymer network in another pH range. Several stitching polymers are selected to create strong adhesion between hydrogels in full range of pH, as well as between hydrogels and various porcine tissues (liver, heart, artery, skin, and stomach). The adhesion energy above 1000 J m−2is achieved when the stitching polymer network elicits the hysteresis in the wet materials. The molecular suture can be designed to be permanent, transient, or removable on‐demand. The topological adhesion may open many opportunities in complex and diverse environments.
When polyelectrolytes and oppositely-charged multivalent ions are mixed in aqueous solutions, they can self-assemble into an array of soft materials and complex fluids, ranging from micro- and nanoparticles, to coacervates, to macroscopic gels. Here, we describe the formation and useful/interesting properties of two such materials: (1) submicron particles formed via ionotropic gelation of the cationic polysaccharide chitosan with tripolyphosphate (TPP); and (2) coacervates prepared from mixtures of the synthetic polycation poly(allylamine hydrochloride) (PAH) with either TPP or pyrophosphate (PPi). For chitosan/TPP particles (which are widely explored as potential drug carriers) we show how, by inhibiting chitosan/TPP binding, monovalent salt (NaCl) can be used to: (1) drastically slow down the rapid ionotropic gelation process to facilitate the experimental analysis of how these particles form; (2) enhance the stability of these particles to aggregation; and (3) achieve improved control over particle size. Unlike the gel-like chitosan/TPP ionic networks, which are both soft (with 10^3 - 10^4 Pa storage moduli) and water-rich, mixtures of PAH with TPP and PPi form high-modulus, putty-like coacervates with storage moduli above 10^5 Pa and much lower (26 - 40 wt%) water contents. These moduli and water contents evidently reflect the high ionic crosslink densities enabled by the densely-charged and flexible PAH chains, and strong PAH/PPi and PAH/TPP binding (which also imparts these coacervates with long relaxation times). Besides their bulk properties, we show that the coacervates adhere to diverse substrates (both hydrophilic and hydrophobic) and, when used as wet adhesives, deliver short-term tensile adhesion strengths above 10^5 Pa. Further, the dense crosslinking within PAH/PPi and PAH/TPP coacervates makes them strong barriers to solute diffusion and (regardless of the solute-coacervate binding strength) enables them to release small water-soluble molecules over multiple months. These findings suggest that PAH/PPi and PAH/TPP coacervates can provide a simple route to both underwater adhesion and long-term controlled release.more » « less
Biomimetic and Bioinspired designs have been investigated due to the advances in modeling, mechanics and experimental characterization of structural features of living organisms. To accomplish bioinspiration for fields such as robotics, adhesives and smart materials, it is required to comprehend how Nature accomplished enhanced mechanical behavior. Among the plethora of complex organisms spanning at different lengthscales, the deep sea sponge Euplectella Aspergillum has been of particular interest due to its lattice structure that can be the framework to design mechanical metamaterials. However, despite its intriguing morphology, constraints in the fabrication and modeling of scalable and nonuniform materials has hindered the study of its mechanical performance and how to harness it. Moreover, a comprehensive FEA model that encompasses the whole spectrum of its constitutive and structural performance has not been reported. In this study, it is aimed to characterize and model the mechanical behavior of this sponge from a structural standpoint. Utilizing various experimental techniques, an FEA mechanical model is developed to study the nonlinear buckling analysis of the sponge’s lattice structure and its resilience to failure. Finally, through topology optimization and sensitivity analysis, a new mechanical metamaterial is proposed. Our results elucidate how mechanical characterization and FEA modeling can be employed for a deeper understanding of Nature’s tailored hierarchy and the design of metamaterials.more » « less