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August 17, 2015

C 2015 American Chemical Society

Wearable and Implantable MechanicalEnergy Harvesters for Self-PoweredBiomedical SystemsRonan Hinchet† and Sang-Woo Kim*,†,‡

†School of Advanced Materials Science and Engineering and ‡SKKU Advanced Institute of Nanotechnology (SAINT), Center for Human Interface Nanotechnology (HINT),Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea

The recent development of small bio-medical systems and microelectronicdevices for healthcare and medical

applications1,2 illustrates the power and thepotential of such a futuristic concept. Possi-ble applications are vast, but they needreliable and safe energy sources to operate.Especially in the fields of biology and med-icine, batteries can be dangerous, bulky,and difficult to change;for example, repla-cing a pacemaker battery can be proble-matic. However, great progress has beenmade in energy harvesters over the pastdecade. In the body, where there is littlelight and there are only small thermal gra-dients, mechanical energy harvesting isneeded to power new medical devices. Bio-medical systems are consuming less andless power, and new harvesting technolo-gies like piezoelectric3 and triboelectric4

nanogenerators have the potential to sup-ply the power needed for the safe operationof medical devices.

MOTIVATION AND OVERVIEW

New Medical Microelectronics Technology. Newbiomedical systems have enormous poten-tial and are motivating active research.These technologies are smaller and more

autonomous and, therefore, are wearable orimplantable. Thus, every patient with oneof these microelectronic devices would beable to receive personalized treatment thatis adapted to their case. The medical possi-bilities are booming; this new technologyhas enormous potential and could offernew control, diagnostic, and treatmentpossibilities.

The first applications developed weremainly aimed at controlling malfunctionsof the body or restoring lost function (e.g.,pacemakers, cochlear implants, retinal im-plants, brain implants, neurostimulators,and prosthesis controllers). These applica-tions opened new horizons for integratingnew features into the body, such as radiofrequency identification (RFID) and near-field communication (NFC) microchips. Inaddition, diagnostic microchips have alsobeen developed to facilitate medical testsat lower cost. These are lab-on-a-chip andmicro total analysis systems (μTAS). Recentresearch demonstrated the benefit of in vivobiomedical systems to treat,5 to help,6,7

and to accelerate8 the healing process ofbody tissue. Patches can fulfill many roles,including disinfectingwounds,7,9monitoringsome patient parameters,10 and transmitting

* Address correspondence tokimsw1@skku.edu.

Published online10.1021/acsnano.5b04855

ABSTRACT In this issue of ACS Nano, Tang et al. investigate the ability of a triboelectric

nanogenerator (TENG) to self-power a low-level laser cure system for osteogenesis by studying the

efficiency of a bone remodeling laser treatment that is powered by a skin-patch-like TENG instead

of a battery. We outline this field by highlighting the motivations for self-powered biomedical

systems and by discussing recent progress in nanogenerators. We note the overlap between

biomedical devices and TENGs and their dawning synergy, and we highlight key prospects for

future developments. Biomedical systems should be more autonomous. This advance could

improve their body integration and fields of action, leading to new medical diagnostics and

treatments. However, future self-powered biomedical systems will need to be more flexible,

biocompatible, and biodegradable. These advances hold the promise of enabling new smart autonomous biomedical systems and contributing significantly

to the Internet of Things.

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this information.11,12 More impress-ively, in this issue of ACS Nano,Tang et al.8 show that wearable,self-powered laser treatments canaccelerate bone remodeling.

Energy Rations. Biomedical sys-tems are intensely researched andare quickly evolving new functions.However, they are also progress-ing toward more integration ontoand into the body, which requiresautonomous systems. These de-vices need power to operate, anddespite recent advances, batteriesstill take up a lot of space, canbe dangerous, and have short-to-medium lifetimes. In addition, be-cause of the strict constraints inbiomedicine, batteries' use of riskymaterials like heavy metals hasmade them even more difficult tointegrate into biomedical systems.Finally, the need to control and toreplace batteries periodically gener-ates necessary maintenance, ofteninvolving surgery, which increasesthe number of critical steps and risks.Therefore, there is a growing needfor new power sources for indepen-dent and continuous operation ofbiomedical systems. Importantly,small-sized microelectronics oper-ating with ultra-low-power con-sumption may be able to powerbiomedical devices by using bodyenergy. This is one way to achieveself-powered implantable systems,and that is why the developmentof energy harvesters has recentlydrawn much attention. Energy har-vesters first aim to complementbatteries and next will developself-powered biomedical devices

for ex vivo and in vivo medicalapplications.

Harvesting the Body's Mechanical En-ergy. Among the different types ofenergy that we can harvest, thereare little light and only small ther-mal gradients in the human body. Itis full of various forms of chemicalenergy like glucose and adenosinetriphosphate (ATP), but their useremains questionable. However,the body is a fabulous source ofmechanical energy. It is a wide-spread product of our muscles andabundant, which makes it a goodenergy source to power autono-mous biomedical systems. To har-vest mechanical energy, diversetechnologies have been developed.In particular, recent emphasis hasbeen placed on the use of nano-materials and nanotechnologies fol-lowing the development of piezo-electric nanogenerators (PENG) in2006.3 They can be efficient, butthey are complex to fabricate anddifficult to integrate. Nevertheless,in 2012, triboelectric nanogenera-tors (TENG)4 were invented andquickly developed on the basis ofelectrostatic and contact electrifica-tion (also named triboelectrification)physics.13,14 Triboelectric nanogen-erators present numerous advan-tages, including high output voltage,efficiency, low cost, high versatility,simplicity in structural design andfabrication, stability and robustness,and environmental friendliness.15

They also enable designs and struc-tures that are robust, less troublesome,and better suited to harvest bodymovements. They facilitate the inte-gration of suchdevices intowearableand implantablebiomedical systems.

For all these reasons, flexible TENGshave recently received particularinterest.

One in vivo experiment has beenaccomplished in a rat by using asmall and flexible planar TENG.16

The device structure is basic and in-expensive, made of amicropatternedpolydimethylsiloxane (PDMS) layer,aluminum (Al) foil, and flexible poly-ethylene terephthalate (PET) spacers(Figure 1a), and its operation is sim-ple. When compressed, the foils (Aland PDMS/PET) are bent betweenthe spacers and touch, generat-ing opposite triboelectric chargesat their surfaces. Then, when theymove, they create electrostatic in-duction, producing an electrical sig-nal to supply power to any small,low-consumption device. The goalof Zheng et al. was to power ahuman pacemaker. The prototypefabricated was small (1.2 cm �1.2 cm, Figure 1b) to fit in vivo inrats. The PET spacers (400 μm thick)fully enclosed the gap in the deviceto protect the inner structure fromthe surrounding bioenvironment. Inaddition, the entire device was cov-ered with a thin and flexible PDMSlayer (50 μm thick) as the encapsula-tion material because of its flexibility,resistance to leaks, biocompatibility,andmild inflammatory reaction uponimplantation. Moreover, this polymerlayer protects the device from bio-fluids and increases its robustness.Because of the spacers and all ofthe packaging, the active surface ofthe device was estimated to be0.8 cm � 0.8 cm. The open-circuitvoltage reached 12 V, and the short-circuit current was 0.25mA, which isequivalent to a power density of0.844 μW 3 cm

�2. For realistic char-acterization of the planar flexibleTENG as an in vivo pacemaker en-ergy source, it was implanted underthe thoracic skin of a rat (Figure 1c)to serve as a breathing energyharvester. The open-circuit voltagegenerated by the breathing of therat was 3.73 V, and the short-circuitcurrent was 0.14 mA. This voltage issufficient to power a commercialpacemaker. Because of the low

Therefore, there is a

growing need for new

power sources for

independent and

continuous operation

of biomedical systems

In this issue of ACS

Nano, Tang et al.

show that wearable,

self-powered laser

treatments can

accelerate bone

remodeling.

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output current, five breaths are ne-cessary to activate a commercialpacemaker. However, consideringthe small size of the planar flexibleTENG due to the rat's size, implant-ing a larger device in the humanbody should allow a commercialpacemaker to be powered witheach breath.

Concerning ex vivo TENG de-vices and patches, a recent paperproposed a TENG skin patch17 toharvest energy from skin deforma-tion. The patch can be rather large,up to 40 cm2, and was attachedto the skin with tape. Here againthe substrate used was PDMS(Figure 1d) because of its stretchingability. The PDMS is a good materialwith which to embed biomedicaldevices because it is biocompatible,inexpensive, easy to prepare, andmore robust than other stretchablemedical polymers like Ecoflex. Toensure that the electrodes hadlow resistance and high stretchingcapacity, serpentine-patterned cop-per electrodes were deposited onthe flexible substrate (Figure 1e).Under stretching (up to 22%) orcompression, the TENG can generatean open-circuit voltage and short-circuit current up to 700 V and 75 μA,respectively. Under realistic situations,

when bending the elbow, knee(Figure 1f), neck, or bicep, it cangenerate tens of volts, which is en-ough energy to power low-energy-consumption biomedical devices.

It is in this important and en-couraging context that Tang et al.8

demonstrate that flexible TENGsattached to human articulationscan generate enough electricityto supply periodic power to a med-ical laser. The aim is for this laserto accelerate bone remodelingtreatments.8 Of course, such a flex-ible TENG equipped with a smallcapacitance as an energy bufferdoes not have the same ability topower a device over time comparedto a battery. The specifications andbehaviors of TENGs are different:batteries deliver continuous poweruntil they are empty, while flexibleTENGs deliver a pulse of powerevery time they are activated.Therefore, the performance of thepowered medical laser varies, suchas producing a continuous lightemission during 60 swhen poweredby a battery, compared to a series of100 light pulses when powered by aflexible TENG. As a consequence, wemight expect significantly differentmedical results. However, Tang et al.show that even if the laser powered

by the TENG is a little less efficientthan the battery-powered laser, bothtreatments work and have goodresults (Figure 2b�d). This findingis important because it proves thatTENGs with a small energy bufferlike a capacitance can be sufficientto power small biomedical devices.Thus, batteries are not mandatory inthese devices, and instead, TENGs arestrong candidates to power futurebiomedical systems.

OUTLOOK AND FUTURECHALLENGES

Biomedical systems and nano-technologies are revolutionizinghealthcare and medicine; their sy-nergy could be extremely powerful,and they could play key roles innear-term medical technologies.Taking into account the decreasingpower consumption of microchipsand the increasing efficiency ofnanomaterials-based mechanicalenergy harvesters such as PENGsand TENGs, it should be possibleto power autonomous biomedicalsystems. Reaching energetic inde-pendence from batteries is a firstimportant step for their develop-ment. However, to reach their fullpotential in the field of healthcare,autonomous biomedical systems

Figure 1. (a) Schematic structure of an implantable planar flexible triboelectric nanogenerator (TENG). (b) Photo of a TENGbefore and (c) after implantation into a rat under its thoracic skin. (d) Schematic structure of the flexible TENG skin patch.(e) Photos of the serpentine-patterned electrodes on human skin and (f) under elbow and knee stretching. Reproduced withpermission from refs 16 and 17. Copyright 2014 and 2015, respectively, Wiley.

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need to continue to evolve moreflexibility and biocompatibility tobe fully operational in the humanbody.To be easily used ex vivo and

in vivo, future autonomous biome-dical systems will need to be wear-able and implantable. In order toachieve this, they will need to befully flexible to fit the shape oforgans,6 including skin, closely.11,12

Thus, the devices will be more dis-crete and comfortable, which is im-portant for the patient, and will bebetter adapted to their target, whichwill increase their sensing ability andthe amount of energy harvested.For example, to sense the heart,it is better to fit its shape6 in orderto get higher mechanical deforma-tions and, thus, better sensing andenergy harvesting. This trend hasalready started and is progressingquickly. Flexible electronics are cur-rently used, flexible microelectro-nics are under development,6,11,12

and flexible TENGs are progress-ing.18 Therefore, flexible autono-mous biomedical devices are possi-ble, and the challenge will be tointegrate all the parts into one small,flexible device. Then the device willneed to be embedded into a flex-ible, biocompatible package to bepatched on or implanted into apatient. Improving and facilitatingthis last step is another importantgoal for biomedical systems.

Conflict of Interest: The authors de-clare no competing financial interest.

Acknowledgment. We acknowledgefinancial support from the IndustrialStrategic Technology Development Pro-gram (10052668) funded by the Ministryof Trade, Industry & Energy (MI, Korea)and the Basic Science Research Program(2009-0083540) through the National

Research Foundation (NRF) of KoreaGrantfunded by the Ministry of Science, ICT &Future Planning.

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Figure 2. (a) Schematic illustration of a flexible arch-shaped triboelectric nanogenerator (TENG). (b�d) Images reveal that thequantity of calcium nodules and mineralized area in both TENG-lased and battery-lased irradiation groups are larger thanthose in the reference group. Reprinted from ref 8. Copyright 2015 American Chemical Society.

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