Piezoelectric generators are based on thin-films of structured materials that can convert pressure into electricity. Inorganic crystals such as aluminum-nitride (AlN) barium titanate (BT) and lead-zirconium-titanate (PZT) have long been explored as piezoelectric films for various applications. Now researchers Sujoy Kumar Ghosh and Dipankar Mandal at the Jadavpur University in Kolkata, India have shown that fish scales (FSC) can be used to build a flexible bio-piezoelectric nanogenerator (BPNG) capable of producing a maximum output power density of 1.14 μW/cm2 under repeated compressive normal stress of 0.17 MPa.

Fabrication of flexible (BPNG) from bio-waste fish-scale (FSC) a) photographs of the bio-waste raw FSC, and demineralised FSC, b) flexibility of the BPNG shown by human fingers, and c) schematic diagram of simple BPNG device structure. (Source: APL)

As recently published in Applied Physics Letters with the title, “High-performance bio-piezoelectric nanogenerator made with fish scale,” the Figure shows that they started with the skin trimmings of Indian carp (Catla catla), that were acid washed and demineralized to extract collagen of nominal thickness ∼250 ± 10 μm, which is then sandwiched between 90nm thick sputtered gold electrodes, followed by lamination with polypropylene (PP) film ∼125 μm thick.
Energy harvesting is enabled by the self-assembled and ordered collagen nano-fibrils, which exhibit intrinsic piezoelectric strength of −5.0 pC/N.

…the most abundant piezoelectric biomaterial present in animal tissues such as skin, tendon, cartilage, bone, and even in human heart, is the type I collagen, which is a biocompatible and biodegradable polymer enabling fabrication of the flexible BPNG. The cost effective collagen source is the fish constituents such as skin, fins, maws, and swim bladder which are mainly treated as “bio waste” materials because different fish species are consumed daily in large quantities worldwide. The disposal of these bio-wastes causes an increasing environmental pollution. The recycling of the fish by-products into the BPNG via one step process is a promising solution for the development of value-added products and also to reduce the e-waste elements.

The BPNG is able to convert several forms of mechanical energy into electricity. For example, gentle press-hold-release motions of a single human finger (∼3.75 kPa and strain rate of 0.017% s−1) results in ∼680 mV output voltage with perfect switching of polarity. It scavenges mechanical energy from high level vibration from machines and also from very low level vibration, arises from sound (∼0.2–2.0 Pa) and wind motions (∼3.6 m/s). When slapped repeatedly by a human hand (∼ 0.17 MPa with 0.77% s−1 strain rate) this BPNG generates a rectified open circuit voltage (Voc) of 4 V, and multiple layers can be stacked to multiply the Voc. Due to high sensitivity, good stability, and efficient piezoelectric power-generating performance, these BPNG may open a new era in sustainable energy harvesting.

—E.K.