Breathe, and a nanogenerator will power your pacemaker

(Nanowerk Spotlight) In a Nanowerk Spotlight from a year ago, we reported on research that showed that high performance piezoelectric ceramics PZT (lead zirconate titanate) could be printed as nanoribbons onto biocompatible and flexible substrates for applications such as harvesting energy from human motion like walking or breathing ("Electricity-generating silicone implants could power electronic devices"). While some motions, such as walking, only require flexibility, others, such as breathing, require that the materials be not just flexible but also stretchable.

However, the PZT ribbons cannot stand stretching operation modes due to their brittle nature, which leads to cracking. The research team, led by Michael McAlpine at Princeton University and Prashant Purohit at the University of Pennsylvania, therefore has been looking to overcome this difficulty by fashioning the piezoelectric ribbons into wavy shapes, and integrating them with stretchable silicone rubber, such that the composite material can withstand large amounts of elastic strain.

Reporting their findings in the February 15, 2011, online edition of Nano Letters ("Enhanced Piezoelectricity and Stretchability in Energy Harvesting Devices Fabricated from Buckled PZT Ribbons") the researchers, which were funded by DARPA and by the NSF, report that they have succeeded in fabricating stretchable nanothick ribbons of piezoelectric PZT.

Stretchable piezoelectric PZT ribbons. (Image: McAlpine Group, Princeton University)

"PZT is among the most efficient piezoelectric materials known, but it is an extremely brittle material, with a Young's Modulus half that of steel," McAlpine explains to Nanowerk. "Thus, the maximum safe strain for PZT is 0.2%, which means even small amounts of stretching will break them. By specially designing the PZT ribbons' shape into a wavy structure, it can be stretched up to 10% strain."

The team's ability to fashion the PZT ribbons' shape into wavy and buckled structures was based on their ability to control prestrains in the silicone rubber and the adhesion between the rubber and the PZT nanoribbons. Specifically, the silicone was pre-stretched, and then the PZT nanoribbons were printed onto the silicone. After printing, the pre-strain was released, forcing the ribbons to buckle up off the surface in ways that could be rationally engineered via calculations performed by Purohit's team.

To fabricate wavy ribbon geometries on soft substrates, the researchers first patterned PZT ribbons (5-10 µm wide and 250-500 nm thick) on a magnesium oxide host substrate as described in the above-mentioned Nanowerk Spotlight, and subsequently released it from the mother substrate using phosphoric acid.

"We then elastically stretched a slab of PDMS (∼ 2mm thick) and brought it into conformal contact with the ribbons" explains Yi Qi, a post doc researcher in McAlpine's group and first author of the paper. "Peeling off the PDMS allowed for complete transfer of the PZT ribbons to the elastomer via adhesive van der Waals forces in the surface-dominated ribbons. Finally, releasing the prestrain in the PDMS led to a compressive force in the PZT ribbons as the PDMS relaxed to zero strain, leading to periodic de-adhesion and buckling. The resulting wavy geometry is a result of the transfer of mechanical compressive energy into bending energy."

Formation of wavy/buckled piezoelectric PZT ribbons. (a) From top to bottom: PZT ribbons were patterned on an MgO substrate and undercut etched to release them from the mother substrate; a slab of prestrained PDMS was laminated against the ribbons and peeled off quickly; retrieved PZT ribbons were transferred onto PDMS and formed wavy/buckled structures upon strain relaxation. (b) SEM image of PZT ribbons transfer printed to PDMS with zero prestrain. (c) PZT ribbons spontaneously buckled under prestrained conditions. (Reprinted with permission from American Chemical Society)

The team found that the wavy shapes of the ribbons can accommodate order-of-magnitude larger post-strains relative to their flat counterparts and thus are suitable for implementation in devices with challenging form factors.

"A bonus feature was that the strain imparted to the ribbons actually led to an increased piezoelectric response due to an effect known as the flexoelectric effect," says McAlpine.

He notes that a particularly fascinating application would be the integration of these highly efficient energy harvesting materials with lungs to power implantable medical devices, such as pacemakers.

"Pacemakers are currently run off of batteries, which means that surgery is required every few years to replace the battery. The lungs operate in a mode similar to a balloon expanding and contracting i.e., in a stretching operation mode. Our biocompatible energy harvesting materials have now been shown to generate power in such stretching modes."

Before we can expect to see such real-life applications, the researchers must overcome the challenge of scaling up their wavy and buckled PZT nanoribbons from the few-ribbon level to entire wafer scales.

McAlpine says that overcoming these fundamental materials integration challenges should simultaneously help to solve the broader challenges of being able to generate sufficient power to actually power implanted biomedical devices from these stretchable energy harvesters.