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Fiber based piezoelectric generators with the ease of integration into the garments can directly convert mechanical energy into electrical signals. The surface charges can be transferred and

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collected by harvesting interface circuit. In the single fiber structure in order to collect the charge from the piezoelectric part electrodes should be in coaxial or core-shell structures. To achieve this approach conductive materials as electrodes need to be coated ²⁶² or wrapped ²⁶³ around active piezoelectric part.

Figure 1.12 Comparison difference types of fiber-base piezoelectric polymer: a) A cylinder shaped of piezoelectric P(VDF-TrFE) fibre with CPC/indium as electrodes and poly(carbonate) (PC) as protective layer.²⁶⁴ b) A flexible piezoelectric fiber from electrospun PVDF-TrFE web.²⁶³ c) SEM image of the PVDF nanofiber membrane and the PVDF/rGO electrode.²⁶⁵ d) Conceptual illustration of design of piezofilm yarn sensor.²⁶⁶ e) SEM image of the cross-section of a single PVDF filament with conductive core electrod.⁴ f) A triaxial braid structure from melt-spun PVDF filament and conductive yarn.⁷¹

By simple rolling and coating method a four stage process piezoelectric fiber was made, electrospun mats were prepared from PVDF–TrFE manually wrapped around a silver coated piezoelectric polymer solution can be coated on conductive substrate such as metal deposited polymer fibers and metal wires but issues such as the poor adhesion between polymer and

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electrodes need to be resolved.²⁶⁷ nylon multifilament yarn that acted as the inner electrode and the wrapped carbon nanotube sheets were outer.

The fabrication of fiber generators were finalised by dip coating in an eleastometric layer which was mechanically protective and electrically insulator.²⁶⁸ Although this structure demonstrate good mechanical and piezoelectric performance but still have slidable connection between the electrodes and the electrospun mat leads to contact instability and increased wearing, thereby negatively affecting the output stability and durability of the devices.²⁶⁹ Also multiple processing steps for the production is time and cost consuming. Another improved method towards achieving a high-performance fiber in a one-step continuous method is a core shell structure which conductive material as an electrode in core and shell fabricated the same time of piezo part fabrication. Piezoelectric generator presented in a three-layered structure which consist of PVDF electrospun web sandwiched between two membranes layer of PVDF/rGO through a continuous process.²⁶⁵

In this technique there is a robust adhesion between the electrodes and the PVDF nanofiber that provide more polarization possibilities. Electrospinning method is not appropriate for mass production and electrospun fibers cannot be used in conventional textile industry machine to fabricate wearable textile. To solve this problem melt spinning technique with the advantage of mass production, high forming quality and on time forming has been widely used.⁸¹ The bicomponent fibres produced with PVDF as core sheath and carbon black/polyethylene as core which are fed through two separate extruders during spinning process. Silver paste on the yarn surface consider as outer electrode.⁴

Since the assembly process has a key role in performance enhancement of the piezoelectric devices, a major aim of the present study was to explore textile processing methods for making such devices. As described in more detail in Chapter three, we have produced a triaxial melt

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spun PVDF fiber has been developed with flexible yarn as electrodes. First, the as-spun PVDF filaments were braided around silver coated nylon yarn as inner electrodes and then the whole structure was covered with braided silver coated nylon fibers as outer electrodes. The developed fiber improved mechanical (i.e. flexibility, comfort and durability) and piezoelectric (i.e. power output and sensitivity) properties.⁷¹

All-fiber piezoelectric energy harvesting devices, encapsulated by a protective layer to increase their durability and wearable facility. Moreover, this technique has attracted significant attention of researchers due to great feasibility of light weight design and also try to use all fiber base generators for the wearable technology.²⁷⁰ Figure 1.12 represent all fiber based piezoelectric generator without using metal wires or foil as electrodes. Fiber-based energy generators have been developed in fabrics form particularly. The piezoelectric performance of the single fiber generators owing to the number and active area limitation are relatively low.

One approach to overcome this problem is integration piezoelectric fibers into fabrics by using diverse textile fabrication technique including knitting, braiding, and weaving (Figure 1.13).

Since wearable energy harvesting devices need to provide right feel and comfort to the wearer, so the best design of the device is integration in the textile as part even whole of the cloth.

Therefore, textile-fiber structures would provide perfect building elements for a smart garment, as they could be naturally integrated into fabrics during the fabrication process without affecting flexibility, comfort and air permeability .²⁰ The soft and flexible fiber-based generators can be designed for the high fatigue resistance under numerous deformation cycles.²⁷¹ Fabric generators should be stretchable same as bending ability to certify a fitting,

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Figure 1.13 Comparison of different types of wearable fabric-base piezoelectric polymer: a) fibers woven into a textile from core-shell structure of PVDF filament.²⁷² b) a 3D piezoelectric fabric from PVDF filament in the knit structure.⁸¹ c) woven flexible textile structure from elastic tubes and piezoelectric film bands with electrods on both sides.²⁷³ d) PVDF/AlO‐rGO flexible nanocomposite.²⁷⁴ e) A woven piezoelectric fabric from twisted yarns of electrospun PVDF-TrFE.²⁷⁵ f) A 3D interlock woven structure established from PVDF and conductive fibers.⁷⁰

improve comfortability and increase the accessibility in human motions for the recovery of energy. High strain elasticity of stretch fabrics associated with knitted fabric stretching and bending of elastic fibers, such as elastane, which offers a restoring force.

Flexibility and stretchability are needed for a strong piezoelectric fiber to be incorporated into smart textiles.²⁶³ The power output from the woven fabric of polyamide yarn and melt-spun PVDF filament showed that the harvested energy from the size 15 × 100 mm of this textile is sufficient to power low-power electronics.²⁷²

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The output voltage of 3D interlock woven fabric from 100% melt-spin piezoelectric filament was 16 times higher than the output voltage provided by a 2D plain woven.⁷⁰ The 3D spacer piezoelectric fabrics have the advantage of efficient charge collection due to proper electrodes connection and also pressure uniform distribution on the fabric surface, causing performance enhancement. Also, this all fibre piezoelectric fabric can be cut into any size and shape without affecting on its flexibility and provide a simple route for integrating.⁸¹ Generally, 3D fabric structures in compare with 2D textiles structures have higher efficiencies because of piezoelectric fibers density.²

Many researchers described flexible wearable generators, however many of them especially in case of electrospinning were not flexible and did not bring the right feel of comfort for the wearers. In fact, they have metal/metallic electrodes which limits their use, flexibility and lifetime.⁸¹ The poor resistance to fatigue causes early failure of the metal foils electrodes. The separation in the metal-insulator-metal structure is due to huge mismatch between the Young's modulus and Poisson's ratio of the metal electrode and the piezoelectric layer. Therefore, loss of mechanical integrity and electric connectivity occurred during long time device operations.²⁷¹ In addition to piezoelectric materials for wearable technology, generate energy can be achieved with triboelectric materials. Triboelectricity through friction, convert mechanical energy to electricity, by a coupled effect of triboelectrification and electrostatic induction.⁷⁶

With appropriate structure designs and using effect of both piezoelectric and triboelectric effect the contact friction and the deformation of the materials can generate voltage and the applied mechanical energy is exploited with high efficiency (Figure 1.14). The current methods to design and fabricate triboelectric structures are based on reactive ion etching or photolithography which both of them are expensive and complicated.²⁷⁶ The electrospinning is a simple, versatile and low-cost method to fabricate ultrathin fibers from a wide range of

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materials as well as ceramics, polymers and composites which most triboelectric generators are made by this technique.⁶⁸

Figure 1.14 Improving performance of fiber base generators by using piezoelectric and triboelectric materials: a) Schematic of the flexible structure of the TENG-based insole.⁶⁸ b) Diagram of a triboelectric energy harvester fabricated by the electrospinning process.²⁷⁷ c) Schematic of stretchable triboelectric structure and SEM image of its component.²⁷⁸ d) Schematic diagram of all-fiber hybrid-triboelectric nanogenerator with two conductive fabrics