Nanocellulose based piezoelectric sensors

SUMMARY
We report the experimental results on piezoelectricity of nanocellulose films. The piezoelectric sensor elements were prepared usign a nanocellulose film as a sensing layer. The piezoelectric sensitivity values from 2-7 pC/N were obtained for the prepared sensor elements.

KEYWORDS: Nanocellulose, piezoelectric sensor, cellulose nanofibrils, polyvinylidenefluoride

INTRODUCTION
Cellulose based nanomaterials, generally known as nanocellulose [1], are interesting renewable bio-based nanomaterials which have potential applications in material sciences, electronics and biomedical engineering and diagnostic. The piezoelectricity of wood initiates from the highly crystalline assemblies of cellulose chains [2]. Experimental evidence of the piezoelectricity of cellulose nanocrystals (CNC) was reported only very recently [3,4]. Cellulose nanofibrils (CNF), produced by a mechanical homogenizing process from cellulose fibers, contain both crystalline and amorphous regions (see Figure 1). CNC can be made from CNF by removal of amorphous regions using hydrolysis.

EXPERIMENTAL
The piezoelectric sensitivity of prepared sensor elements is measured using in-house built measurement setup equipped with a mechanical shaker and charge amplifier (See Figure 2). The setup is described elsewhere in details [5].

RESULTS AND DISCUSSION
A randomly oriented CNF film (prepared by pressure filtering from aqueous CNF dispersion) showed piezoelectric sensitivities of 2-7 pC/N [6,7]. The values align between the piezoelectric coefficients of quartz (2.3 pC/N) and polyvinylidenefluoride (PVDF, -30 pC/N).

CONCLUSION
In this work, we have successfully demonstrated the fabrication of piezoelectric sensors using nanocellulose as a sensing material. Based on these findings, nanocellulose seems a promising material for sensors and energy harvesting applications.

REFERENCES
[1] Moon, R. J., et al. "Cellulose nanomaterials review: structure, properties and nanocomposites." Chemical Society Reviews 40.7 (2011): 3941.
[2] Fukada, E. "Piezoelectricity of wood." Journal of the Physical Society of Japan 10.2 (1955): 149.
[3] Csoka, L. et al. "Piezoelectric effect of cellulose nanocrystals thin films." ACS Macro Letters 1.7 (2012): 867.
[4] Frka-Petesic, B. et al. "First experimental evidence of a giant permanent electric-dipole moment in cellulose nanocrystals." Europhysics Letters 107.2 (2014): 28006.
[5] Rajala, S. et al. “Structural and Electrical Characterization of Solution-Processed Electrodes for Piezoelectric Polymer Film Sensors.” IEEE Sensors Journal (Accepted for publication 2015).
[6] Rajala, S. et al. "Piezoelectric sensitivity measurements of cellulose nanofibrils sensors." Proc. XXI IMEKO World Congr., Prague, Czech Republic (2015).
[7] Tuukkanen, S. et al. "A survey of printable piezoelectric sensors." In: Proceedings of IEEE Sensors 2015 Conference, (2015): 1426.