3-D Printable Flexible Hydrogel-Based Sensors With Gradient Porous Structure

Hydrogel-based flexible pressure sensors have been intensively studied to detect human physiological activities. Here, a kind of hydrogel with excellent 3-D printability has been developed. The gelation method based on the freeze-thaw cycles of polyvinyl alcohol (PVA) and the addition of (TEMPO)-oxidized cellulose nanofibrils (TOCNF) as rheological modifier rendered the printed hydrogel patterns great morphological fidelity. In addition, the conductivity of the hydrogels is as high as 3.8 S/m with the promotion of TOCNF. Also, the prepared hydrogels have gratifying mechanical properties under the salting-out effect (tensile and compressive stresses increased by 527% and 460%, respectively). The printed hydrogels with porous structure were applied to the electrode and dielectric layer of capacitance sensors (the dielectric layer was wrapped with ultrathin polyethylene (PE) film). The porous structure effectively improved the sensitivity of the pressure sensors (from 0.023 and 0.182 to 0.045 and 0.689 kPa $^{-1}$ ) when used as electrode and dielectric layer, respectively), while the gradient porous structure further broadened the linearity range of high sensitivity (electrode, from 2.92 to 15.35 kPa; dielectric layer, from 11.21 to 39.51 kPa). The results indicated that gradient porous structure is a simple and effective strategy to enhance both the sensitivity and linearity range of hydrogel-based sensors. The hydrogel-based sensors with printed structure have stable and sensitive capacitance and resistance response, and can also be used for motion monitoring of body parts, such as finger and arm.