Table 4.

Ambient Energy Harvesters Used in Flexible Sensing Systems^a

Energy source	Mechanism	Sensing function	Peak power density	Flexible form factor	Ref
Movement (touch, friction, bending, stretching, impact, vibration, sound & ultrasound, etc.)	Piezoelectric (also for acoustic wireless power transfer)	Strain, pressure, pulse	3–9.3 kW m−3	Sheets, (ultra)thin films	685-687
			1.25 W m−2	Fibers, fabrics (stretchable)	688,689
			26 mW m−2	Stretchable sponges	690
	Triboelectric	Touch, pressure, strain, breath, vibration, sound, acceleration, gas, humidity	Up to 10 MW m−2	Sheets, sponges	691,692
			4.3 μW m−1, 1.77 W m−2, 230 mW m−2	Fibers, yarns, fabrics (stretchable)	689,693-695
			35 mW m−2	(Self-healable) stretchable slabs	696,697
	Piezoelectric + triboelectric	Touch, bending, stepping	2.34 W m−2	Stretchable fabrics	698
	Magnetoelastic	Pulse, breath, touch, pressure, bending, stepping	20–43.2 W m−2	Stretchable slabs, sheets	65,68,169,208
			6.67 W m−2	Stretchable fibers and fabrics	66,67
	Magnetoelectrical	Arm waving, strain	3.2 W m−2	Yarns, fabrics	699
			Peak power: 20 μW	Stretchable yarn and ring	700
	Electrostatic	Stretch, twist, compression	0.55 mW m−2	Stretchable slabs	701
Radiation (sunlight, light emitting diode, laser, etc.)	Photovoltaic	No	11.5 W g−1 (PCEb 10.5%), 4.4 W g−1 (PCE 5.8%), 180 W m−2 (PCE ~19%)	(Ultra)thin films	676,682,702,703
			2.53 mW m−1, 0.61 W m−2 (PCE 11.9%)	Fibers, fabrics	678,704
Heat	Thermoelectric	Temperature	111 W m−2 at 50 K ΔT, 188 W m−2 at 80 K ΔT	Sheets, films	705,706
			6.5 W m−2 at 60-K ΔT	Stretchable slabs	707
			110 W m−2 at 70-K ΔT, 23 W m−2 at 60-K ΔT	Fibers, fabrics	708-711
	Thermocell	No	0.6–0.7 mW m−2 K−2	Stretchable slabs	712,713
Water (moisture, humidity, sweat)	Hydrovoltaic	No	760 μW m−2 at 25% RHc and 55.2 mW m−2 at 85% RH	Sheets	714
			~10 mW m−2 immersion in DI waterd	Thin films	715
			~70 W m−3 (= 21 mW m−2 at 0.3 mm thickness) at ~80% RH	Fabrics	716
	Hydrovoltaic + thermoelectric	No	720 mW m−2 at 70% RH and 10 K ΔT	Sheets	717
	Hydrovoltaic + photovoltaic	No	880 mW m−2 at 50% RH and 1 sun illumination	Sheets	718
Biofluids (sweat, blood, interstitial fluid)	Biofuel cell (on-skin)	Glucose, lactate, etc.	35 W m−2	Integrated skin patches	719
			83 mW m−2	Integrated sensor chips	720
			130 mW m−2 at 30 mM lactate	Integrated skin patches	232
			12 W m−2	Stretchable sheets	721
	Biofuel cell (implantable)	Glucose, lactate, etc.	44 mW m−2 on implantation and 25 mW m−2 after a month	Fibers	722
			430 mW m−2 in 0.5 M glucose	Nanomembrane on Si substrate	723
			4.8 W m−2 in 5 mM glucose and 1.6 W m−2 after 7 days	Bioresorbable sheets	724
	CNT-OH interaction (implantable)	No	570 mW g−1 in 2.5 M HCl	Fibers	725

^aSome state-of-the-art reports are listed.

^bPCE, power conversion efficiency.

^cRH, relative humidity.

^dDI water, deionized water.