Cellulose Hydrogel Wearable Sensors Work at -25°C for a Week

At first sight, cotton pulp doesn’t seem to have a role in the future of wearable electronics; it is the fluffy material left behind when raw cotton is processed, a byproduct of a long-established industry. However, researchers from Southwest Jiaotong University in China have made it into something far more interesting - a flexible, transparent hydrogel which is able to detect human movement, can be printed into desired shapes using a 3D printer, and continues to function properly at -25°C for a whole week without its performance dropping.

The article in the Journal of Bioresources and Bioproducts looks at one of the ongoing frustrations encountered in the development of wearable sensors. Since hydrogels are soft materials rich in water and therefore make good interfaces between electronics and human skin, they have a fundamental physical issue: water freezes. Upon freezing, the hydrogel becomes stiff, swells and loses the ionic conductivity which enables it to transmit electrical signals. If you take a typical hydrogel sensor and use it skiing, it ends up being an expensive wristband, as Interesting Engineering has reported.

Cellulose hydrogel wearable sensors

The answer is in the chemistry of the solvent system. The researchers dissolved cellulose obtained from cotton pulp in a mixture of two salts - zinc chloride and lithium bromide - which were combined to form a molten salt hydrate. This is not ordinary salt water; the zinc chloride and lithium bromide ions interact strongly with water molecules and thus disrupt the hydrogen bonding networks that enable water to form ice crystals. Measurements carried out using differential scanning calorimetry, a method which records the heat flow as a material is cooled, revealed no exothermic peaks in the temperature range from -80°C to 20°C. To put it simply, the water contained within the hydrogel just won’t freeze, even when the temperature is well below that which a household freezer can achieve.

The identical solvent system also dissolves the cellulose itself, since cotton pulp cellulose has a closely packed crystalline structure that is held together by hydrogen bonds between its polymer chains. These hydrogen bonds are broken by the salt mixture and the cellulose molecules disperse, only to re-form a continuous gel network as the solution cools. The end product is a hydrogel known as HZ0.3L0.7-C3 which is transparent, mechanically strong, and electrically conductive.

The figures are solid: the ionic conductivity amounted to 4.48 siemens per metre, which is high enough to enable the transmission of clear electrical signals when the material is mechanically deformed. The compressive strength was 2.48 megapascals, enough to tolerate repeated pressing without causing any permanent damage. Following seven consecutive days at -25°C, the hydrogel gave rise to repeatable electrical signals upon being bent or pressed, with no measurable loss in sensitivity.

Sensors for the human body

The material also exhibits shear-thinning behaviour, a characteristic which means that it is suitable for use in extrusion-based 3D printing; under the pressure exerted by the print nozzle the hydrogel flows but once it has been deposited it maintains its shape. The researchers proved this by printing out structures such as five-pointed stars and maple leaf shapes, thus demonstrating that the material can be formed into shapes tailored to particular sensing applications.

To improve its biocompatibility for use on the body, the team applied a polydopamine coating. Sensors attached to the fingers, wrists, elbows, and throats were able to detect bending, pressing, and vibration. The response time was about 100 milliseconds and the recovery time roughly 300 milliseconds. The hydrogel kept its stable performance after 500 compression cycles at a strain of 30%.

As an example of its practical potential, the researchers constructed a data-glove system in which hydrogel sensors on each finger picked up movements of the hand and sent them in real time to a robotic model. The robot reproduced the operator’s gestures, thus creating a simple human-machine interface made completely of materials derived from cotton.

The research is still at an experimental stage and the path from laboratory prototype to a commercial product is both lengthy and uncertain. There are still unanswered questions concerning long-term durability, the scalability of manufacturing, and the need for regulatory approval of medical devices that come into contact with the skin. Yet the fundamental idea - that a plentiful natural polymer can be turned into a freeze-proof, printable, body-compatible sensor - indicates that the next generation of wearable electronics could be grown in a field before it is eventually worn on the wrist.