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Stretchable Piezoelectric Nanogenerators for Energy Harvesting in Elastic Environments

Periodic Reporting for period 1 - SPENG (Stretchable Piezoelectric Nanogenerators for Energy Harvesting in Elastic Environments)

Reporting period: 2016-08-01 to 2018-07-31

Nanoscale piezoelectric (PZ) energy harvesters, or nanogenerators (NGs), are vital for next-generation autonomous devices as they can directly convert small-scale vibrations, such as blood flow and body movements, into electrical energy. Scavenging power from ubiquitous vibrations in this way offers an attractive route to supersede fixed power sources such as batteries that need constant replacing/recharging. In particular, epidermal or implantable PZ NGs could revolutionize wearable electronics and healthcare monitoring. The associated elastic environments require not only flexibility of the NG, but also stretchability in order for it to remain operational. Current NGs are rarely functional without being coupled to rigid or, at best, flexible substrates, due to the lack of proper methodology for fabrication of both stretchable electrodes as well as stretchable high-performance PZ nanomaterials, that together make up PZ NGs. Thus, the Action has aimed to (i) develop micro/ nano-patterned electrode fabrication techniques based on electronic printing on flexible/stretchable substrates, (ii) develop polymer-based PZ materials with tailored elastic properties to satisfy stretchability and flexibility criteria, marking a departure from traditional PZ materials that are ceramic in nature and hence stiff and brittle, and (iii) study the efficiency of the stretchable NGs developed, based on simulations and direct measurements of energy harvesting (EH) performance in elastic environments.
In this Action, the Researcher has firstly successfully designed and fabricated freestanding stretchable electrodes, based on aerosol jet printing technology, conductive nanoparticle inks and polymeric inks. The electrode was geometrically structured to be stretchable, providing up to 180 % extension with resistance being changed by less than 3 times. The as-fabricated device could potentially be applied as a strain sensor under large deformation. Multi-layered of electrodes can be fabricated over one stretchable structure, making it applicable in on-skin touching sensing, integrated stretchable circuit, or stretchable humidity sensing. The Researcher has also conducted reviewing on polymer-based PZ NGs, simulation performing of the as-fabricated electrodes being applied in power generating, as well as attempt of integrating the nanogenerator with the electrode. Due to time limitation, the fully integrated stretchable PZ NG was not accomplished due to the as-fabricated stretchable electrodes not being structurally preferable in driving PZ material, based on some simulation and experimental results. Overall, the achieved objectives in this Action have addressed fabrication techniques for stretchable electrodes and their conducting or sensing applications in biological and other extreme environments that involve large scale deformation, as well as to provide a potential way, which still needs more research, to integrate polymeric PZ materials for stretchable energy harvesters.
There have been 2 work packages for this Action.
In Work Package A the researcher has successfully design and fabricated a freestanding stretchable electrode based on modern aerosol jet printing (AJP) technique. Work performed included i) to become a skilled user of aerosol jet printer for complicate fabrication requirement, ii) ink development for electrode printing, iii) electric and mechanical characterization of the fabricated electrodes, and iv) optimization of the fabrication strategy and structure design.
In Work Package B the researcher has attempted to integrate the as-fabricated electrode with polymeric piezoelectric nanogenerators. Work conducted included i) a review of current soft material based nanogenerators, ii) polymeric piezoelectric nanowire characterization, iii) simulation and experimental study on mechanical performance for double layered stretchable electrode for applying in stretchable nanogenerators, iv) applications and demonstrations development for single or double layered stretchable electrode.
The fulfilment of Work Package B had some deviation from its original objectives. However, the researcher had established some interesting applications and demonstrations in related to the results achieved from Work Package A. The results from the Action have been published as various deliverables, including 3 conference presentations (2 oral, 1 poster) and 9 journal papers (including 1 currently being under review.) The technique of fabricating stretchable electrode developed in this Action will be further adopted in future researches, with one currently being conducted to monitor the strain of large deformation in extreme conditions of heart valve. The results of the Action will be further exposed to academic world with more and more applications being developed based on them.
Originally the stretchable electrode was planned to be designed for applied in piezoelectric energy harvester. Usually such electrodes were fabricated with elastic supporting substrate. However according to the tests from this Action such existence limited the strechability of the electrode itself. The researcher developed a freestanding stretchable electrode to overcome this weakness. Moreover, the structure could have multiple layers of electrode stack together. Especially the two-layered stretchable electrode was not only a potential solution for integrating polymeric piezoelectric materials as dielectric layers for nanogenerators, its unique integrated properties were also good for epidermal touching sensing, stretchable circuit and stretchable humidity sensing. The idea could be expended from energy harvester to all electronics which provides a lot of opportunities in integrated stretchable electronics such as sensors, LEDs, batteries, etc. This work has led to an ongoing collaboration with researchers studying annuloplasty implantable rings to treat mitral regurgitation which is a serious heart condition. The printed stretchable strain sensors developed in this Action are being cistomised for in situ sensing of the forces and deformation of the implantable rings during cardiac cycles, thereby providing large amounts of data to help accelerate clinical trials and scientific research in this field.
Figure summarises the fabrication, characterization & application of the stretchable electrode