Binary mixtures of liquid metal (LM) or low-melting-point alloy (LMPA) in an elastomeric or fluidic carrier medium can exhibit unique combinations of electrical, thermal, and mechanical properties. This emerging class of soft multifunctional composites have potential applications in wearable computing, bio-inspired robotics, and shape-programmable architectures. The dispersion phase can range from dilute droplets to connected networks that support electrical conductivity. In contrast to deterministically patterned LM microfluidics, LMPA- and LM-embedded elastomer (LMEE) composites are statistically homogenous and exhibit effective bulk properties. Eutectic Ga-In (EGaIn) and Ga-In-Sn (Galinstan) alloys are typically used due to their high conductivity, low viscosity, negligible nontoxicity, and ability to wet to nonmetallic materials. Because they are liquid-phase, these alloys can alter the electrical and thermal properties of the composite while preserving the mechanics of the surrounding medium. For composites with LMPA inclusions (e.g., Field's metal, Pb-based solder), mechanical rigidity can be actively tuned with external heating or electrical activation. This progress report, reviews recent experimental and theoretical studies of this emerging class of soft material architectures and identifies current technical challenges and opportunities for further advancement.

Soft Multifunctional Composites and Emulsions with Liquid Metals

Materials with switchable mechanical properties are widespread in living organisms and endow many species with traits that are essential for their survival. Many of the mechanically morphing materials systems found in nature are based on hierarchical structures, which are the basis for mechanical robustness and often also the key to responsive behavior. Many “operating principles” involve cascades of events that translate cues from the environment into changes of the overall structure and/or the connectivity of the constituting building blocks at various levels. These concepts permit dramatic property variations without significant compositional changes. Inspired by the function and the growing understanding of the operating principles at play in biological materials with the capability to change their mechanical properties, significant efforts have been made toward mimicking such architectures and functions in artificial materials. Research in this domain has rapidly grown in the last two decades and aff...

Bioinspired Polymer Systems with Stimuli-Responsive Mechanical Properties

http://doc.rero.ch/record/17562/files/wed_bma.pdf

Biomimetic mechanically adaptive nanocomposites

The principal component of the body wall of the sea cucumber Cucumaria frondosa is a dermis consisting of collagen fibrils, microfibrils, proteoglycans and other soluble and insoluble components. A major structural constituent of the dermis is a network of 10­14 nm diameter microfibrils, which surrounds and penetrates bundles of collagen fibrils. This network has been extracted and purified using guanidine and bacterial collagenase. Tensile testing of the microfibrillar network in artificial sea water demonstrates that it is reversibly extensible up to approximately 300 % of its initial length. It behaves like a viscoelastic solid, having a long-range elastic component as well as a time-dependent viscous component. Reduction and alkylation of the cysteine residues in the network do not change its breaking strain or strength, but greatly increase the compliance of the network until, near the breaking strain, the tensile resistance rapidly increases. These data suggest that the strength of the network is due to non-reducible crosslinks, while its elasticity is dependent upon disulfide bonds. In deionized water, the network becomes swollen and, although it remains elastic, is much more compliant than when tested in artificial sea water. Examination of whole tissues and purified networks with the electron microscope reveals structures similar to vertebrate fibrillin-containing microfibrils. Considering that the dermis of C. frondosa is a mechanically mutable tissue in which elongation is accompanied by the sliding of collagen fibrils past one another, the microfibrillar network may act to maintain the orientation of fibrillar components during movement and may also provide a long-range restoring force.

Morphology and biomechanics of the microfibrillar network of sea cucumber dermis

http://web.mit.edu/hyneslab/pdfs/The%20Echinoderm%20adhesome.pdf

The echinoderm adhesome

Summary

(1) Catch connective tissue is defined as the collagenous connective tissue whose mechanical properties can be changed rapidly (in seconds or minutes) under nervous control.



(2) Catch connective tissues are found in all five classes of Echinodermata. They function in tone control of the tissues and in autotomy.



(3) The change in mechanical properties occurs in viscosity.



(4) Muscle cells are not responsible for the viscosity change.



(5) The viscosity change is controlled by nervous activities. Neurosecretory-like cells with large electron-dense granules are found in all the catch connective tissues so far studied.



(6) The viscosity change is quite likely caused by the change in the ionic environment in the connective tissues, which alters the weak (non-covalent) interactions between extracellular macromolecules in the tissue.

Connective tissue catch in echinoderms

The stiffness change of the holothurian dermis caused by chemical and electrical stimulation.

1.Stress-relaxation tests and creep tests were performed on the body-wall dermis of two sea cucumbers, Actinopyga echinites (Jager) and Holothuria leucospilota Brandt. 2.These viscoelastic connective tissues had mechanical properties which agreed well with those of a four-element mechanical model composed of two Maxwell elements connected in parallel. 3.The elastic stiffness of the dermis of Actinopyga was 1.7 MPa and that of Holothuria was 042 MPa. 4.The viscosity of the dermis showed great variation of more than two orders. 5.Chemical stimulation with artificial sea water containing 100 mM potassium increased the viscosity but not elasticity. 6.The viscosity change is suggested to be caused by the change in weak (non-covalent) bonds between macromolecules which constitute the dermis.

/pdf/viscoelasticity-of-holothurian-body-wall-477gv2irmm.pdf

Viscoelasticity of Holothurian Body Wall

In the regular echinoid Diadema setosum Leske the central ligament that connects the spine to its tubercle is mainly composed of closely-packed collagen fibres which are arranged in the longitudinal axis of the ligament The mechanical properties of the ligament are quite different from ligament to ligament: the viscosity determined by creep tests ranges from 0·02 to 6 GPa·s The viscosity of a ligament changes reversibly in response to stimulation Adrenaline and noradrenaline (10-6-10-3 M) decrease the viscosity Acetylcholine (10-8-10-3 M), artificial sea water containing high (100 MM) potassium concentration, and electrical stimulation increase the viscosity As the ligament contains no muscle cells in it, the viscosity change cannot be attributable to muscle activities The functional significance of having a central ligament with variable and controllable viscosity is that it binds the spine base onto the articulation surface of the tubercle, so as to reduce the possibility of spine dislocation, while being flexible enough to allow the spine considerable freedom of movement when necessary

Tatsuo Motokawa

Papers

Connective tissue catch in echinoderms

The stiffness change of the holothurian dermis caused by chemical and electrical stimulation.

Viscoelasticity of Holothurian Body Wall

Mechanical properties and structure of the spine‐joint central ligament of the sea urchin, Diadema setosum (Echinodermata, Echinoidea)

Cholinergic control of the mechanical properties of the catch connective tissue in the holothurian body wall.