Energy autonomous electronic skin

“Dye-sensitized solar cells (DSSC) use a combination of interpenetrating networks based on mesoscopic and wide bandgap semiconductor materials as photo-electrodes with dye-sensitizer electrolytes as light absorbing material electrolytes.(69) DSSCs mainly differ from conventional semiconductor solid-state PV cells in te…”

“The batteries offer a limited life span and short charging/discharging cyclic stability and durability, risky overheating effects, and are often heavy.(26) Because of the need, and currently the lack of suitable solutions, significant efforts have been devoted during the last decades to develop alternative solutions s…”

“Quantum-dots sensitized solar cells (QDSSCs) have emerged rapidly in the field of PVs mainly due to their unique optoelectronic properties which can be accurately tailored by either varying QDs dimensions or doping processes.(85) Their design which is similar to DSSCs, includes deposition of narrow bandgap semiconduct…”

“Strain-induced charge generated by a single piezoelectric ZnO NW, in clamped-free configuration, has been demonstrated by contacting the tip of atomic force microscope (AFM) on the top of NW vertically aligned on the substrate surface.(101) These results exhibit energy discharges of around 0.”

“Triboelectric energy nanogenerators (TENGs) were discovered from “badly” encapsulated ZnO NWs piezoelectric energy generators, where bottom and top electrodes were allowed to slide or contact and the electrostatic induction during the process leading to generation of few volts from triboelectric effect.(37) TENGs are…”

“Organic PVs (OPVs) are mainly based on four different structures, including single-layer, bi-layer heterojunction, bulk heterojunction and diffuse-layer heterojunction.(63) The lowest performance obtained by OPVs is shown by single-layer and bi-layer heterojunction, which present a poor PCE below 1% and maximum PD of…”

“9% and a PD of around 118W/m(2).(73) These achievements are promising for powering micro- and nano-devices integrated in an e-skin.”

“4 at 300 K).(133) Chalcogenide compounds such as Cs1−xRbxBi4Te, have also been used as TE materials for near-/mid-room-temperature applications, e.”

“7 W/ m(2).(79) On the other hand, bulk heterojunction OPV cells exhibit a better performance, showing higher PCE up to 5–8%, and PD of 49.”

“However, this mechanism is now being explored to transform mechanical energy to electricity to drive a wide range of applications.(46,47,121) A large number of natural systems inherently present motions (e.”

“In the field of PV cells, the most promising approaches that are being explored are PV cells based on quantum dots(210) and perovskite materials.(65,89) With respect to the technologies based on mechanical energy harvesting, tribo-electricity has opened new lines of investigation offering higher efficiencies than thos…”

“87 Further improvements have been achieved by doping QDs with metallic impurities (e.g. Mn2+ doping of CdS), showing record PCE of 5.42% and PD up to 54.3 W/m2.”

“5% (and PD of 26 W/m(2)) for conjugated polymer/methanofullerene blend.(82) Moreover, comparing the power per mass unit (W/g) of well-stablished PV technologies such as a-Si (0.”

“Among all, GaInP/GaAs MJ-based PV cells have shown the highest PCE of 46% and exhibited PD up to 345W/m(2) (Table 1).(66) In spite of the excellent performance demonstrated by these PV cells, their implantation on e-skin applications is not straightforward yet because of their poor mechanical properties, non-compatibi…”

“35 V).(111,112) These works demonstrate sufficient generation of energy required to power nano- and micro-devices with power consumption in the range of μW–mW.”

“As examples of achievements in the field, WPT based on NFC have been successfully utilized in both bio-implantable chips(183) and electric vehicles.(184) These achievements are promising for the self-powered e-skin applications covered in this work, where distance between different elements distributed/stacked along t…”

“Considering a NW density of 20 NWs/μm(2), the power density by multiple ZnO NWs vertically aligned on a substrate under similar conditions could be about 10 nanostructures based on sensors.(108,109) Similar experiments have been carried out on different kind of semiconductor NWs, including ZnO (output voltage ~8mV),(1…”

“) and developing new approaches to improve their integration over largeareas.(39,188,207,208) In the initial phase of e-skin development the use of low-power electronics was a matter of intensive investigation.”

“Reprinted with permission from Dahiya et al.50 Copyright © 2013, IEEE. i Image of the prosthetic/robotic hand with e-skin.8 npj Flexible Electronics (2019) 1 Published in partnership with Nanjing Tech University 1 2 3 4 5 6 7 8 9 0 () :,; with the increasing number of sensing and associated electronic components.”

“for e-skin applications on humans or animals) are also crucial properties for the use of PVs as energy source in portable and wearable selfpowered e-skin.(61) For the sake of comparison, the PCE (obtained using an A.”

“3a–c) to transform the output characteristic of the resulting generator from DC to alternatingcurrent (AC) and thus showing a significant increase of the generated power.(103) Thus, the output characteristic of aforementioned ZnO NWs-based piezoelectric generators overcome those issues with conventional PZT for self-p…”

“For example, a micro-generator (~0.1 cm3) consisting of four magnets integrated on an etched cantilever with a wound coil located within the moving magnetic field has been shown to exhibit output powers of 46 µW (0.46 mW/cm3) using a VAC of 0.48 V and an input excitation frequency of 64 Hz.100 The design of such energ…”

“a surgery tool with a e-skin would allow the surgeon to get a complete feedback from the patient tissue during the operation.(193) In this section, we have compiled the most relevant works, reporting the successful integration of both energy harvesting and storing technologies, forming a hybrid system capable to suppl…”

“WPT has also attracted significant attention in the field of wearable selfpowered systems, and particularly in e-skin applications, where some preliminary results have demonstrated promising results.(217) However, drawbacks such as low distance range of the energy transfer, screening effects in the environment, or slo…”

“For example, a micro-generator (~0.1 cm3) consisting of four magnets integrated on an etched cantilever with a wound coil located within the moving magnetic field has been shown to exhibit output powers of 46 µW (0.46 mW/cm3) using a VAC of 0.48 V and an input excitation frequency of 64 Hz.100 The design of such energ…”

“).(121,127) The tremendous advances achieved in nano-fabrication technologies allows nowadays to expect a significant reduction in the size of components (e.”

“Reprinted with permission from Guo et al.158 Copyright © 2016, John Wiley and Sons. d Photograph (left) and 3D schema (right) of CNT-based stretchable SC. Reprinted with permission from Meng et al.161 Copyright © 2010, American Chemical Society. e 3D schema and photograph (inset) of a flexible SC packaged using plasti…”

“With respect to the technologies based on mechanical energy harvesting, tribo-electricity has opened new lines of investigation offering higher efficiencies than those obtained by conventional piezoelectric generators.(44,45,122) In this regard, triboelectric generators have demonstrated the potential to harvest that…”

“) and developing new approaches to improve their integration over largeareas.(39,188,207,208) In the initial phase of e-skin development the use of low-power electronics was a matter of intensive investigation.”

“These values are around 2–3 orders of magnitudes higher than the output from commercial vibrational microgenrators.(27) The output of these piezoelectric generators can be further improved (e.”

“Organic and inorganic metal perovskite solar cells (PSCs) have emerged as the promising candidates for solarenergy harvesting with excellent PCE and PD.(35,65) The excellent PSCs, combined with features such as flexibility and lightweight, PSCs offer great alternative to current flexible PVs (based on Si, IIIV, organi…”

“Mechanical vibrations available in the ambient have wide range of frequencies, from few Hz (human steps, human heartbeat rate, and sea water waves) to several kHz (mechanical engines), and can produce energy in a wide range of few hundreds μWcm to mWcm.(94) Mechanical energy is helpful to ensure the continuous operati…”

“87 Further improvements have been achieved by doping QDs with metallic impurities (e.g. Mn2+ doping of CdS), showing record PCE of 5.42% and PD up to 54.3 W/m2.”

“However, a number of alternative hole-transport material (HTM) such as redox mediators and electrolytes have also been investigated, including: (i) I/I in solid polymer, gel, ionic liquid, or plastic crystal systems; (ii) solid inorganic materials; (iii) Co/Co and SeCN/(SeCN) redox couples; (iv) and hole-conducting or…”

“Another approach, also based on AC piezoelectric generators, consists of cyclic stretching/releasing of a single piezoelectric fine wire (PFW) integrated on a flexible substrate.(117) The periodic bending/stretching of the PFW shows record output voltages and energy conversion efficiencies of 50mV and 6.”

“In this regard, the fabrication of flexible SCs has been demonstrated by integrating electrochemically active materials such as metal oxides and conducting polymers with flexible fibres made of graphene, carbon, metal and plastic.(163,164)”

“Some of the above energy-harvesting technologies, which were covered in this review, have already reached the market, and have been successfully developed on flexible/stretchable and lightweight substrates.(28,37,41,102) This new technology will offer an excellent opportunity for the development of self-powered system…”

“5 at elevated temperatures (>900 K).(130) Several new structures and materials have also been explored recently.”

“3 V and the generated energy successfully used to power a lightemitting diode (LED).(115) ZnO NWs with a tapered structure (e.”

“The dielectric-ondielectric in lateral sliding mode TENGs show that the in-plane sliding of two dielectric materials can also exhibit triboelectric effect.(123) This approach has been demonstrated to generate power densities up to 5.”

“7d).(160) State-of-art SCs are generally made of conductive polymers (Fig.”

“With respect to the technologies based on mechanical energy harvesting, tribo-electricity has opened new lines of investigation offering higher efficiencies than those obtained by conventional piezoelectric generators.(44,45,122) In this regard, triboelectric generators have demonstrated the potential to harvest that…”

“3 Piezoelectric energy generators. a 3D schema of a single NW-based piezoelectric nanogenerator fabricated on a flexible substrate and subjected to b stretching and c compression conditions. d 3D schema presenting the working principle of a self-powered touch nano-sensors powered by a piezoelectric nanogenerator a–c.…”

“Reprinted with permission from Fu et al.(164) Copyright © 2012, John Wiley and Sons.”

“6 V piezopotential along the NW cross-section when external force is applied at the free end of the NW.(105) This piezopotential is enough to drive metal−semiconductor Schottky diode.”

“Another example of a promising ongoing investigation has recently demonstrated a prototype of large-area electronic carpet capable to harvest energy from human steps.(97) These technologies are relevant for e-skin, as mechanical energy could be harvested by robots or human movements to power the distributed electric c…”

“4g).(125) The TENGs based on rotation mode are also based on inplane charge separation like aforementioned sliding modes, with some additional advantages such as energy harvesting from rotational motions.”

“87 Further improvements have been achieved by doping QDs with metallic impurities (e.g. Mn2+ doping of CdS), showing record PCE of 5.42% and PD up to 54.3 W/m2.”

“The BFCs are classified based on their biochemical reactions and the nature of the electrodes.(148) The microbial and enzymatic-based BFCs have exhibited record values of performance (micro-BFC: 10–24 μW/cm(2);(32) enzymatic BFC: 32 μW/cm(2)).”

“V), wearable sensors self-powered by thermoelectric generators (reprinted with permission from Leonov et al.54 Copyright © 2009, AIP Publishing), wearable e-patch for human health monitoring (reprinted with permission from Yang et al.55 Copyright © 2009, American Chemical Society), multi-sensing e-skin on fabrics self…”

“6% and a PD of 276W/m(2).(67) These are promising results to power micro-/nano-electronics (see the section 'Self-powered e-skin'), for example, in a human inspired e-skin for humanoid (effective area: 1.”

“Some of the above energy-harvesting technologies, which were covered in this review, have already reached the market, and have been successfully developed on flexible/stretchable and lightweight substrates.(28,37,41,102) This new technology will offer an excellent opportunity for the development of self-powered system…”

“3f).(55,118) In this scenario, the mechanical energy produced by a NW-based nanogenerator attached to either the body of a rat (Fig.”

“09%, however, also showing clear cracking of the perovskite layer under bending conditions.(93) Further advances are needed to ensure the mechanical stability and flexural endurance of PSCs towards large-area flexible PVs.”

“63 at 1100 K is predicted, with S of 45 and 300 μV/K measured at 300 and 900 K, respectively.(132) Both, p- and n-type semiconductor materials with different structures have been investigated for high-temperature TE applications.”

“Reprinted with permissions from Lee et al.166 Copyright © 2013, American Chemical Society npj Flexible Electronics (2019) 1 Published in partnership with Nanjing Tech University Accordingly, the strategies that are being followed today include using new materials and configurations of the battery components, i.e. curr…”

“3 W/ m(2).(87) Further improvements have been achieved by doping QDs with metallic impurities (e.”

“BFCs have also demonstrated great applicability as they meet such requirements due to their biocompatibility, flexibility/stretchability, and scalability towards high-power-density generation, compatibility to soft substrate to minimize the impact on skin irritation and wearers comfort.(153,154) For such wearable syst…”

“Some of the above energy-harvesting technologies, which were covered in this review, have already reached the market, and have been successfully developed on flexible/stretchable and lightweight substrates.(28,37,41,102) This new technology will offer an excellent opportunity for the development of self-powered system…”

“Reprinted with permission from Cannata et al.(49,53) The 3D reconstruction of a skin patch placed on g a container, and h a KUKA LWR arm.”

“This makes them promising candidates as energy harvesters for self-powered wearable systems on flexible substrates.(129,135) The NWs-based superlattices, InSb quantum-dot structures, Si NWs, single wall carbon nanotube (SWCNT) bundle, BiTe NWs and a GaAs NWs, are predicted to have better thermoelectric properties than…”

“).(121,127) The tremendous advances achieved in nano-fabrication technologies allows nowadays to expect a significant reduction in the size of components (e.”

“Flexible fibre-shaped ZAB knitted into clothes and textiles have been successfully reported to exhibit energy densities of around 649mWh/g.(173) The connection of three fibre-shaped ZABs knitted in series holds a potential of 3.”

“LiBs have also been made from LTO and LMO nanoparticles integrated into two aligned MWCNT yarns, which act as anode and cathode.(171) The fabricated wire-shaped LiBs exhibit energy densities of 27Wh/kg (17.”

“Wireless powering of sensors is currently an intensive matter of investigation.(31) The current advances related to the design and development of sensors on non-conventional flexible/stretchable substrates, have opened promising alternatives to distribute more number of thin, soft and skin-like sensors along human or…”

“For example, ZT values greater than 4 and 6 are predicted for 5-nm diameter PbSe/PbS and PbTe/PbSe superlattice NWs at 77 K.(136) Alternatively, tubular nanostructures such as nanotubes (NTs), have also demonstrated a great potential as TE materials.”

“Considering a NW density of 20 NWs/μm(2), the power density by multiple ZnO NWs vertically aligned on a substrate under similar conditions could be about 10 nanostructures based on sensors.(108,109) Similar experiments have been carried out on different kind of semiconductor NWs, including ZnO (output voltage ~8mV),(1…”

“Highly stretchable textile BFCs have been fabricated by screen printing of stretchable inks with CNT and Ag2O/Ag electrodes, exhibiting stable power outputs after 100 cycles of 100% stretching.(156) The fabricated BFCs produces power densities of 160 and 250mW/cm(2) with singleenzyme and membrane-free configurations,…”

“8g).(1) A similar approach followed recently uses touch sensors based on piezoelectronics metal oxide microstructures, self-powered by a perovskite PV cell.”

“35 V).(111,112) These works demonstrate sufficient generation of energy required to power nano- and micro-devices with power consumption in the range of μW–mW.”

“For example, graphene has been used as electronic layer, temperature sensor and pressure sensor, all integrated on the same flexible substrate, reducing the incompatibility of each functional layer.(205) The fabrication of next generation of e-skin involves the design of new architectures to ensure an reliable interac…”

“6%; PD= 66W/m(2)).(74) Metal oxide NWs with optical fibres or planar waveguides,(75) have been shown to exhibit PCE of 3.”

“conical shape, embedded in a polymeric rubber, and sandwiched by two metallic electrodes) can be macroscopic piezoelectric generators (~7000 NWs/mm(2)), with output voltage and power of 2 V and 118 nW, respectively, and capable to power continuously a small crystal liquid display.(116) Another approach, also based on…”

“For example, a micro-generator (~0.1 cm3) consisting of four magnets integrated on an etched cantilever with a wound coil located within the moving magnetic field has been shown to exhibit output powers of 46 µW (0.46 mW/cm3) using a VAC of 0.48 V and an input excitation frequency of 64 Hz.100 The design of such energ…”

“Photographs of a flexible SPP consisting of a flexible PV cell and a LiB wrapped on a f bag and g a travel mug. h 3D schema and i working principle of a pulse oximeter powered by a SPP. Reprinted with permission from Ostfeld et al.195 Copyright © 2016, Springer Nature npj Flexible Electronics (2019) 1 Published in par…”

“triboelectrics), but also on the integration of different energy harvesting and storage technologies, resulting in a portable power pack.(38,48) Prior to the discussion about the potential strategies to harvest, store and deliver energy to e-skin, it is important to mention the power requirements of this technology so…”

“7e),(161) metal oxides nanostructures,(158) carbon-based materials (carbon nanotubes (CNTs) and graphene),(159,160) or hybrid combination of these materials.(157,162) In addition to their excellent energy storage capacity, the SCs are also exceptional because of their quick energy delivery (charging/discharging rates…”

“These microfibres results in output powers ranging between 20 and 80mW/ m(2), which is attractive for wearable systems as they could harvest energy from expansions/compression by movements in human body such as heartbeat pulses and exhaling actions.(120) In this regard, a relevant example is the carbon fibres coated w…”

“In the configuration where NWs are doubly clamped between two electrodes on a flexible substrate, the piezopotential is generated along the c-axis of the NW (straining direction), resulting in values in the range of hundreds of volts.(106) The electron screening effect, sometimes observed at the positive side of the N…”

“Copyright © 2017, John Wiley and Sons), e-skin self-powered by a biofuel cell and integrated on a contact-lens (reprinted with permission from Falk et al.(34) Copyright © 2012 Elsevier B.”

“V), wearable sensors self-powered by thermoelectric generators (reprinted with permission from Leonov et al.54 Copyright © 2009, AIP Publishing), wearable e-patch for human health monitoring (reprinted with permission from Yang et al.55 Copyright © 2009, American Chemical Society), multi-sensing e-skin on fabrics self…”

“discovering new materials), high energy density energy storage devices for implantable applications.(180) Great efforts have been dedicated on the optimization of WPT.”

“Both impedance and resonant frequency characteristic of NFC power transfer systems have been investigated to improve the coupling distance resulting in the minor loss of power.(182) As examples of achievements in the field, WPT based on NFC have been successfully utilized in both bio-implantable chips(183) and electri…”

“Copyright © 2009, American Chemical Society), multi-sensing e-skin on fabrics self-powered by piezoelectric generators (Reprinted with permission from Li et al.(56) Copyright © 2016, Springer Nature) C.”

“10b).(196) The developed unit called self-charging micro-supercapacitor power unit (SCMPU) was inserted in the insole of a shoe, allowing to light up continuously LED during a daily human walking (Fig.”

“when a car stops in a cross road, allowing also to charge wirelessly its batteries.(96) Another example of a promising ongoing investigation has recently demonstrated a prototype of large-area electronic carpet capable to harvest energy from human steps.”

“In most of the aforementioned hybrid technologies, BFCs are based on blood glucose as a fuel, which requires the implantation of the BFCs in the body.(149,150) Nonetheless, non-invasive epidermal BFCs based on temporary transfer tattoos (tBFCs) have been also reported (Fig.”

“In this regard, a promising strategy is to fill the perovskite structure with polymers, preserving the crystallinity of the film, and increasing the reliability of the resulting PSCs over several bending cycles and high bending radii.(65) This approach has shown flexible PSCs with PCE up to 12.”

“Reprinted with permission from Jia et al.152 Copyright © 2014, Royal Society of Chemistry Published in partnership with Nanjing Tech University npj Flexible Electronics (2019) 1 (capacitance change 1% under bending angles of 180°) (Fig.”

“However, much progress is required, especially to design high energy density devices for implantable applications and for high transfer efficiency.(176) The possibility of charging batteries or SCs through WPT technologies will have tremendous advantage for portable and autonomous systems in a broad number of applicat…”

“triboelectrics), but also on the integration of different energy harvesting and storage technologies, resulting in a portable power pack.(38,48) Prior to the discussion about the potential strategies to harvest, store and deliver energy to e-skin, it is important to mention the power requirements of this technology so…”

“5 mAh/cm(2).(174) The experimental process employed to fabricate these cells are compatible with rollto-roll techniques, which makes this technology promising for the low-cost production of self-powered stretchable systems.”

“11 μW/cm(2) and a voltage of 62 mV) is observed by paring the metal coated ZnOnanotip top electrodes with the active ZnO NWs array in a multilayer stack configuration.(113) Such a stacked configuration is well-aligned with 3D e-skin applications, with various layers in the stack having different functionalities (Fig.”

“Alternatively, the progress made in the solutionbased methods to synthesize metal oxide nanostructures such as ZnO NWs at low temperatures (100 °C), makes possible the direct growth of these nanostructures on plastic substrates.(77) This has permitted the development of flexible DSSCs at a very low cost and showing st…”

“By reducing the thermal conductivity and optimizing carrier concentration in bulk materials, the above approaches have demonstrated high ZT of 1.25—at high temperatures around 900 K—and maximum S of 200 μV/K.131 Following this approach, Ba8Ga16Ge30 alloy has been shown to have ZT of 1.35 at 900 K, and an extrapolated…”

“The electron screening effect, sometimes observed at the positive side of the NW during the crystal lattice distortion, shows that the intentional/unintentional n-doping of ZnO NWs could hinder the piezopotential.(107) Nonetheless, a reduction of carrier density in ZnO through engineering approaches have made ZnO NWs…”

“In case of PZNGs, different approaches, including materials and structures, have been studied for SPP.”

“21 V.(155) High energy density and stretchability are the key challenges for this type of biofuels.”

“i Image of the prosthetic/robotic hand with e-skin.(8) C.”

“An example of vibration as the source of mechanical energy, in the application unrelated to eskin, can be found in roads and pedestrian paths where cars and humans steps have been used to produce energy.(95) Energy ranging between 50 and 500 μW can be developed in this way.”

“7e),(161) metal oxides nanostructures,(158) carbon-based materials (carbon nanotubes (CNTs) and graphene),(159,160) or hybrid combination of these materials.(157,162) In addition to their excellent energy storage capacity, the SCs are also exceptional because of their quick energy delivery (charging/discharging rates…”

“These relate to: (i) tailoring mechanical properties,(6,10) (ii) integration of various technologies resulting in a hybrid system,(166,170) (iii) device architecture(200) and packaging,(1) (iv) efficiency and capacity of energy storage technology,(30) (v) low-power electronics to ensure positive energy budget,(8,201)…”

“Reprinted with permission from Bai et al.44 Copyright © 2013, American Chemical Society. d Photograph of hundreds of LEDs lighted up by a TENG and e electric circuit. f Current generated by TENG from a human hand sweeping at different speeds. g Current peaks measured at each state of two groups of LEDs during hand swe…”

“Reprinted with permission from Guo et al.158 Copyright © 2016, John Wiley and Sons. d Photograph (left) and 3D schema (right) of CNT-based stretchable SC. Reprinted with permission from Meng et al.161 Copyright © 2010, American Chemical Society. e 3D schema and photograph (inset) of a flexible SC packaged using plasti…”

“8 W/m(2), which are promising values but still far from the other PV technologies discussed above.(88) Despite of their poor performance with respect to other state-of-art PV cells, the low weight due to the utilization of NPs, combined with lowtemperature processing on non-conventional substrates such as cloth and pa…”

“For example, CNTs-based thin films have been utilized either as current collector with plain paper as a separator (108 mAh/g),(167) or in hybrid electrodes consisting of ultra-long CNTs and V2O5 NWs (169 mAh/g),(168) or in LiMnTiO4/MWCNT-based electrodes (161 mAh/g),(169) and in sodium 1,4-dioxonaphthalene-2-sulfonate…”

“In most of the aforementioned hybrid technologies, BFCs are based on blood glucose as a fuel, which requires the implantation of the BFCs in the body.(149,150) Nonetheless, non-invasive epidermal BFCs based on temporary transfer tattoos (tBFCs) have been also reported (Fig.”