Piezoelectricity refers to the generation of electric potential (voltage) when specific materials are subjected to mechanical stress (Steinem & Janshoff, 2005). This mechanical force can be torsional, compressional, shear or flexural (Wudyet al., 2009). It was first discovered in 1880 by the Curie brothers, Pierre and Paul-Jacques, using a variety of crystals including quartz, tourmaline and Rochelle salt (Britannica, 2018). An electric potential can only be generated ifthe crystalline property contains a non-symmetrical unit cell; the smallestrepeatable unit of volume forming the crystal lattice pattern (Wudy et al., 2009; Mahan, 2021). With the application of mechanical force, the crystalline becomes compressed,resulting in displacement of the ions within the unit cells (Wudy et al., 2009). This creates an electric polarization of the unit cell, which magnifies over the repeating cellsto form a measurable electric potential (Wudy et al., 2009). The piezoelectric effect is also reversible (converse piezoelectric effect) when an external electrical field is applied to crystalline materials, creating a mechanical deformation (Steinem & Janshoff, 2005).
In 1957, Fukada and Yasuda made the discovery that bone can also act in accordance with the piezoelectric effect and the converse piezoelectric effect. Although it was shown to have a reduced piezoelectric effect constant compared to crystals (one tenth of quartz) (Fukada & Yasuda, 1957). In the following years, they also tested the direct and converse piezoelectric effect on tendon (Fukada & Yasuda, 1964). The results showed a similar piezoelectric constant and voltage sensitivity to that of quartz crystal, as well as specific polarization patterns dependant on the angle of force and electric field (Fukada & Yasuda, 1964). The coupling coefficient and acoustic resistance were notably lower in the tendon compared to quartz, due to its smaller elastic modulus, density and sound velocity (Fukada & Yasuda, 1964).
Since Fukada and Yasuda’s findings, electrical stimulation has become an emerging and growing market for the treatment of delayed and non-union fractures, particularly in the United States. When tissues are damaged, they are unable to provide biophysical input to connective tissues at an accurate rate due to disordered electric currents (Kooistra et al., 2009). The notion behind electric stimulation is that it provides added electric and electromagnetic currents through damaged tissue to aid and magnify the physiological response of healing (Kooistra et al., 2009). There are three principle devices used to deliver electrical stimulation when treating fracture sites: direct electrical stimulation, capacitive coupling and inductive coupling (Kuzyk & Schemitsch, 2009). Many in vitro and exploratory case studies have demonstrated notable results regarding bone growth and healing using these three devices (Kooistra et al., 2009). However, meta analyses of randomized trials have found the data to be insufficient in concluding that electrical stimulation is an effective treatment for delayed and non-union fractures (Goldstein et al., 2010; Kooistra et al., 2009). Despite the lack of statistical significance, the study trials display a positive trend favouring electrical stimulation treatment (Goldstein et al., 2010)., indicating that more large-scale randomized trials are needed to justify its efficacy on participants.
Today, there are many applications for piezoelectricity, both commercially and biomedically (Chen-Glasser et al., 2018). Common commercialized applications utilizing the piezoelectric effect are timekeeping, microphones, speakers, fuel injection and hydrophones (Chen-Glasser et al., 2018). In terms of a biomedical context, there are three main types of piezoelectric technology: electric sensing, energy harvesting and surgery (Chen-Glasser et al., 2018). As many human vital signs such as breathing follow a rhythmic pattern, electric sensors work by converting that mechanical energy into an electric signal (Chen-Glasser et al., 2018). Sensors can be used to examine joint movement, brain pressure, breathing rate and much more (Chen-Glasser et al., 2018). Piezoelectricity has made great strides in the field of surgery (Chen-Glasser et al., 2018). It was first used in dentistry with removing implants, detaching the inferior alveolar nerve and bone harvesting (Chen-Glasser et al., 2018). Stacked rings composed of piezoelectric ceramics make up the generic piezosurgical devices (Chen-Glasser et al., 2018). A voltage is applied through the rings and transmitted to the tip of the device as vibrations (Chen-Glasser et al., 2018). A common example of this is the ultrasonic lancet which is used often in delicate surgeries to conserve surrounding tissues and perform micromovements (Chen-Glasser et al., 2018). Energy harvesting is an innovative approach of using the body’s piezoelectricity (generated through movement) to power sensors/monitoring devices (Chen-Glasser et al., 2018). Most piezoelectric biomedical energy harvesting devices are composed of a polymer or ceramic film encased around a consistently moving organ (lung, heart, diaphragm, etc.) (Chen-Glasser et al., 2018). With all of these technologies there are limitations and more research is needed before application outside of an experimental setting (Chen-Glasser et al., 2018).
Britannica. (2018). Piezoelectricity. In Encyclopedia Britannica. Retrieved September 7, 2021, from https://www.britannica.com/science/piezoelectricity.
Goldstein, C., Sprague, S., & Petrisor, B. A. (2010).Electrical stimulation for fracture healing: current evidence. Journal of Orthopaedic Trauma, 24, S62-S65. doi: 10.1097/BOT.0b013e3181cdde1b
Mahan, G.D. (Ed.). (2021). Crystal. In Encyclopedia Britannica. https://www.britannica.com/science/crystal
Steinem, C., & Janshoff, A. (2005). SENSORS |Piezoelectric Resonators. In P. Worsfold, A. Townshend & C. Poole (Eds), Encyclopedia of Analytical Science (2nd ed.). ScienceDirect. https://doi.org/10.1016/B0-12-369397-7/00556-2
Wudy, F., Stock, C., & Gores, H.J. (2009). MEASUREMENT METHODS | Electrochemical: QuartzMicrobalance. In J. Garche (Ed.), Encyclopedia of Electrochemical PowerSources. ScienceDirect. https://doi.org/10.1016/B978-044452745-5.00079-4