Exploring the long-term biocompatibility of implantable nanogenerators
Piezoelectric materials have been used as transducers or to power small-scale sensors for many years. They can harvest even the smallest deflections from the environment, transforming micro- and nanoscale mechanical energy into useable electrical voltages. But it’s their potential for use inside the human body that may prove the largest impact. The first implantable nanogenerator was successfully demonstrated in 2010. Though not much more than a proof-of-concept, it ignited a worldwide race to produce a working, practical power source for the next generation of implantable medical devices.
Studies focused on harvesting heartbeats have been promising, suggesting that piezoelectric nanogenerators affixed to bovine or porcine hearts could produce sufficient voltages to power pacemakers. Other research has shown that nanogenerators could act as ‘active sensors’ that continuously monitor physiological processes, like blood pressure and respiration. Such devices have also been shown to have extensively long lifespans. But there are still many open questions around the long-term bio-compatibility and bio-safety of implantable nanogenerators.
In a paper published in Nano Energy [DOI: 10.1016/j.nanoen.2018.07.008], a team from the University of Wisconsin-Madison have explored the behaviour of polyvinylidenefluoride (PVDF) nanogenerators, both in vitro and in vivo, over a period of six months. The PVDF films were packaged in one of two biocompatible polymers – polydimethylsiloxane (PDMS) or Parylene-C – before being implanted between the skin and muscle layer in the hip joints of young female mice.
The devices were studied in vivo using computed tomography (CT), ultrasound, and photoacoustic imaging. Throughout the entire implantation period (24 weeks), the nanogenerators exhibited excellent structural and functional stability. However, the team found that as the mice moved their hip, the PDMS-encapsulated nanogenerator more exactly followed the shape of the muscle than the Parylene-C device. Given that deflection of the nanogenerator is the key to optimising its electrical output, they concluded that PDMS is a superior packaging material for these devices. In vitro tests showed that both generators produced a stable voltage, even after 7200 cycles.
As well as understanding the stability of device in the biofluid environment, the authors also set out to understand what impact the nanogenerator had on the surrounding tissue. The stray current from the device was shown to be just 0.0001% of the functional current, which suggests that PDMS acts as an effective electrical insulator. Histological, blood and serum studies also revealed no signs of toxicity or incompatibility over the six month study, suggesting that such devices may offer a route to practical biomechanical energy harvesting.