Hydrogels for Soft Machines

Abstract

Hydrogels have applications in surgery and drug delivery, but are never considered alongside polymers and composites as materials for mechanical design. This is because synthetic hydrogels are in general very weak. In contrast, many biological gel composites, such as cartilage, are quite strong, and function as tough, shock‐absorbing structural solids. The recent development of strong hydrogels suggests that it may be possible to design new families of strong gels that would allow the design of soft biomimetic machines, which have not previously been possible.

Highly Stretchable and Transparent Ionogels as Nonvolatile Conductors for Dielectric Elastomer Transducers

“…[1][2][3] A soft machine uses the large deformation of soft materials to assist humans, 4, 5 operate robots, [6][7][8] monitor living tissues, 9,10 sense environment, 11,12 shape light, 13,14 and harvest energy. 15 A technology under intense development is electromechanical transduction using dielectric elastomers.…”

Hybrid Hydrogels with Extremely High Stiffness and Toughness

“…7-9 These load-bearing applications of hydrogels are often limited by low stiffness and toughness. 10 Hydrogels for cartilage replacement, for instance, require high stiffness and toughness to retain shape and to resist fracture, respectively. 5 Several approaches have been reported to improve the toughness of hydrogels, [11][12][13][14] but simultaneously achieving high stiffness and toughness remains a challenge.…”

Morphing in nature and beyond: a review of natural and synthetic shape-changing materials and mechanisms

“…For example, the 20 % of 20 nm collagen fibres found in the cornea are theorised to be the source of its 4 MPa tensile strength [106]. A similar concept was demonstrated with fibre reinforcement of an epoxy-based hydrogel, increasing its breaking stress by a factor of 20 [111]-despite to retain other properties simultaneously, the fibres would need to be significantly smaller, as seen in the cornea.…”

“…However, the recent success of dye-sensitised solar cells, which are achieving commercial success despite currently relying on a corrosive liquid electrolyte, suggests this does not necessarily disqualify a technology [105]. As Calvert has commented, fruits such as oranges require hydration and yet may be transported across the world without issue [106].…”

“…Synthetic hydrogels typically have fracture energies of about 10 Jm -2 ; cartilage, meanwhile, withstands an additional two orders of magnitude, fracturing at around 1000 Jm -2 [106]. Many strategies are currently being employed to strengthen hydrogels, including double networks, composite addition and structural modification, but there is a long way to go before their potential can be fully realised [110].…”