Flexible fish

Wearable electronics and other devices attached to soft surfaces must be sufficiently flexible to stretch and bend in tandem with the movement of their host. Transparent plastics are widely used as flexible substrates for mounting electroluminescent devices and electronic displays but are non-renewable.

Now, researchers in China have introduced an alternative substrate derived from collagen in waste fish scales, which is readily degraded in soil and easily recycled (ACS Nano, doi.org/10.1021/ acsnano.9b09880).

A fish gelatin (FG) solution was prepared by mixing freeze-dried fish scales with water at 80°C and the supernatant was poured into a petri dish and left to dry for 48 hours to form a thin film. Laboratory testing demonstrated that the FG film was transparent, with light transmittance up to 91.1% in the visible spectrum.

An 80μm FG film was also highly flexible and could, for example, be folded reversibly into an ‘origami frog’. Thicker films were less flexible and thinner ones were more prone to wrinkling.

The films dissolve in water and the resulting solution can be re-cast to form another FG film. The process could be repeated many times with no loss of transparency.

‘[However], excellent water solubility may prevent FG-based devices from operating in extreme weather,’ says Hai-Dong Yu, who co-led the research team from Nanjing Tech University. ‘The use of degradable waterproof materials to package the devices might be a solution.’

Importantly, the researchers were able to demonstrate that the FG film completely degrades in soil within 24 days. ‘Regarding the biodegradation of the FG film in soil, it has been reported that out of every 1kg of the film waste, 200g was considered as compost, and residual organic matter was digested by microorganisms within the biodegradation process,’ says Hai-Dong Yu.

The team used the FG film to construct an alternating current electroluminescent (ACEL) device. Silver nanowires were first deposited on the film, resulting in a flexible, transparent and highly conductive electrode. Further integration of a ZnS:Cu, Mn phosphor and capping with polyvinylpyrrolidone allowed the team to build an ACEL device that continued to glow even after being bent and relaxed 1000 times.

Commenting on the potential utility of FG-based electronics, Hai-Dong Yu says: ‘Flexible electronics based on the FG films are suitable for some applications, but not for other applications, due to their excellent water solubility and relatively poor thermal stability. The poor thermal stability of the FG films makes them unable to withstand the temperature required by E-beam evaporation, calcination and other processes, which needs to be optimised. Currently, only the FG film is recyclable, which dominates the weight of the ACEL devices, and we are investigating [ways] to make the entire device recyclable.’