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Williams NX, Franklin AD. Electronic Tattoos: A Promising Approach toReal-time Theragnostics. J Dermatol & Skin Sci. 2020;2(1):5-16Support ElectronicsWithout a system to support and facilitate transductionand communication, the implementation of on-skin sensingand therapy would be all but impossible. Numerousconsiderations must be made to ensure accuracy, comfort,and durability. Human skin can stretch by up to 25% beforeincurring damage14. Given this, to maintain comfort, anepidermal tattoo must be thin enough and have a sufficientlylow modulus ( 1.5 mm thick and 600 kilopascals (kPa))to retain comfort and must maintain performance metricswhen strained15.One major concern is loss of electrical conductivity dueto strain; to alleviate this failure mechanism, numerousmethods have been proposed, starting with materialselection. Recently, significant work has been performedon liquid metals for stretchable conductive electronics16,17.At room temperature, metals such as eutectic indiumgallium maintain a liquid form, which allows conductivechannels containing these materials to be stressed over300% without any noticeable degradation18. This, however,has the drawback of requiring encapsulation, which greatlyreduces utility and versatility. Another materials category,that perhaps is more directly suited to electronic tattoos,are high aspect ratio materials, such as silver nanowires.Their high aspect ratio allows for only a small increasein electrical resistance with applied bending and tensilestrain19,20, and makes them an ideal candidate for electrodesand contacts in an electronic tattoo21.In addition to materials choice, significantimprovements to lifetime and resistance to degradationfrom cyclic strain can be achieved via engineering designof electrodes through the incorporation of a serpentinepath. This approach can increase ultimate strain from aslittle as 1% for some materials to upwards of 300%22. Thisdesign alleviates stress from tensile strain as it allows forthe coiled electrodes to straighten before considerablestress is placed upon the conductive trace itself. To furtherresist degradation, encapsulation can also work to sustainperformance by upwards of 6x after numerous cyclic strainepisodes typical of quotidian movement23.Finally, in addition to conductive traces that arethe backbone of electronics, signal processing andtransmission are required. Most conventional electronicmedical devices separate on- or in-body measurementfrom amplification, signal filtering, processing, andinterpretation; however, in some cases for fully onskin devices, these processes must be miniaturized andincorporated into the electronic tattoo package. Whilethe vast majority of demonstrated electronic tattoos useconventional silicon-based integrated circuits (ICs)19,24,there is a growing body of research developing stretchabletransistors and other stretchable electrical componentsJournal of Dermatology and Skin Sciencefor incorporation into flexible electronics25,26. Currently,the performance of conventional electronics is ordersof magnitude greater and the scale is substantiallysmaller than achievable with flexible components. Thusthe incorporation of flexible circuit boards may requirean increase in footprint as compared to a silicon IC27.However, if conformity to the skin is a requirement, adevice fabricated from all flexible components may bedesired, in which case, the larger area could be less of aconcern.The most likely path for the support electronics will beto first implement only the needed control circuitry andsignal transmission to an external device (e.g., smartphone,as depicted in Figure 1), keeping the complexity on-skinto a minimum; then, in the longer term, implement othersupport electronics into the tattoo when the feasibilityof doing so is realized. Even in the near-term, obstaclesremain for realizing all needed diagnostic and therapeuticcontrol along with signal transmission in an on-skinelectronic tattoo, including in the performance and stabilityof the electronic devices, electrical interconnections andinterfaces, and scalability in cost and size.Diagnostic SensingThere are numerous cases where continuousmonitoring can be advantageous as compared to discretepoint monitoring, not least of which is the removal ofrequired human interfacing, which can be hindered byexhaustion, misuse of tools, and/or access to a clinic.With new technological advancements propelling bothconsumer electronics and medical devices towards aninternet-of-everything (IOE) ideal of connectivity, researchinto electronic biomedical sensors has proliferated. Much ofthe development focus is in on-skin sensors. From its birthin 1999 with the continuous detection of blood glucose28,the field has expanded to include sensing of strain29,30(Figure 2A-B), temperature31 (Figure 2C) and several otherbiological markers related to the exponential increase insweat sensors32,33 in the past several years. These on-skinelectrical sensors can be divided into two equally necessarycategories: physical and chemical sensors.Physical and chemical sensors can be distinguishedby the sensing mechanism. Physical sensors measure anattribute via a change to the sensor itself, whereas chemicalsensors measure a reaction between the target analyteand the sensor. As an example, physical sensors includetemperature sensors and strain sensors. For a temperaturesensor, the sensing mechanism is derived from a change inresistance of the active sensing component in response to amodification in skin temperature. A strain sensor, likewise,transduces the strain experienced by the skin (throughcompression, extension, or torsion) via a proportionalchange in the resistance caused by some physical changeto the sensor. Depending on the sensor itself, this couldPage 7 of 16

Williams NX, Franklin AD. Electronic Tattoos: A Promising Approach toReal-time Theragnostics. J Dermatol & Skin Sci. 2020;2(1):5-16Journal of Dermatology and Skin ScienceFigure 2. Types of electronic tattoo diagnostic sensors. Physical sensors, which use a physical change in the sensor to measure a biologicalsignal, including (A, B) strain sensors and (C) thermal sensors, (D) touch sensors, and (E) electrocardiograms. Chemical sensors measurethe response of chemical reactions such as (F) blood glucose and (G) alcohol. Multiplexed sensors to measure both (H) alcohol & bloodglucose and (I) sweat analytes. Reprinted with permission from: (A) ref. 29, copyright 2019, Advanced Materials; B) ref. 30, copyright2017, Nature Nanotechnology; C) ref. 31, copyright 2019, Advanced Science; D) ref. 12, copyright 2018, Advanced Functional Materials;E) ref. 22, copyright 2019, Advanced Science; F) ref. 34, copyright 2015, Analytical Chemistry; G) ref. 35, copyright 2016, ACS sensors; H)ref. 36, copyright 2018, Advanced Science; I) ref 37, copyright 2019, Science Advances).be a capacitive change as in a touch sensor12 (Figure2D), an increase or decrease to the junction densitybetween electrically conductive components (such assilver nanowires)30, or modulation to energy band gap inthe channel region of a transistor34, among others. Thecommonality between these transduction mechanisms isan internal change to a structure or property of the sensoritself. Hence, an electrocardiogram (ECG), would likewisebe categorized as a physical sensor as the electrical signalis generally measured via a capacitive change22 (Figure2E).Chemical sensors, on the other hand, transduceinformation (most often, electrically) via the responseto a chemical shift. This may be with an oxidoreductasePage 8 of 16

Williams NX, Franklin AD. Electronic Tattoos: A Promising Approach toReal-time Theragnostics. J Dermatol & Skin Sci. 2020;2(1):5-16Journal of Dermatology and Skin Scienceenzymatic reaction (such as glucose oxidation facilitatedby glucose oxidase, as seen in Figure 2F-H)35–37 orvia the binding event of a protein-protein pair (suchas the immunogenic antibody-antigen binding of animmunoassay)38–40. While physical sensors can measuresignals transdermally, chemical sensors require a solute,and hence a bodily fluid. This can be achieved ex vivo viasweat41 (Figure 2I) or interstitial fluid42,43.on a single chip55, yet more research and validation arerequired to extend this development to more devices.For interstitial fluid – the fluid between cells within thebody – cultivation requires microneedles to puncture theepidermis, while sweat sensing can be achieved without anyporation of the skin. As with blood, interstitial fluid is easilyprobed with an implantable device; however, it is difficultto access non-invasively44, whereas sweat can be easilygenerated and measured transdermally. There are twomethods for generation of sweat: natural production33 andthe more common induction via the iontophoretic deliveryof a drug, such as Pilocarpine45. While natural methods maybe ideal for monitoring sweat during exercise, stimulatedperspiration allows for greater control over sweat timingand volume. Sweat generation remains an issue for longterm use as natural techniques cannot be maintaineddue to physical exertion requirements and inducedperspiration uses a limited drug resource on a localizeddevice. Furthermore, due to degradation of the sensingmechanism, much of the testing focuses on rapid detectionon the order of seconds29,35,36,41,46,47, minutes24,37,48,49, andhours50, rather than stability over days of use. To propelchemical sensing development into a commercializablestratum, more focus is needed on extending the lifetime ofsensors from hours to days.Given the relative novelty of this field, many of the reportsare proof-of-concept and thus require further testing todirectly compare to commercial sensors. To overcomethis barrier, significant efforts are currently underway toincrease the sensitivity, selectivity, and reproducibility ofchemical biosensors51. Simultaneous delivery of all three ofthese metrics is required and will reduce anguish causedby false positives and delayed treatment caused by falsenegatives52. One significant hurdle, as of yet not entirelyovercome, is lack of reproducibility in electronic biosensorsat least partially caused by drift during storage or intrinsicdevice-to-device variations53. Furthermore, as the fieldadvances, validation studies comparing electronic tattoosto the gold standards of clinic-based detection to ensure thecorrect and accurate measurement of analytes will becomeimperative. The field is new enough that this morass hasnot yet affected development because much of the work todate focuses on initial, singular demonstrations54; however,more detailed and long-term studies should become amore expected element of future studies. Recent effortshave seen success in on-chip calibration, which comparesthe response from a blank to that of a functionalized deviceIn contrast to chemical sensors, much of the limitationsof current physical sensors are largely related todegradation due to repetitive sensing and to cyclic bending/stretching56–58. As previously stated, human skin can bestrained significantly before damage14. Recent studiesindicate that a 90 wrist flexion can strain the epidermalsurface of the forearm by upwards of 25%59,60. Giventhat these are minor movements that occur continuallythroughout daily motion, cyclic strain reliability is a majorissue. While numerous reports include some cyclic straindata, the majority fall short of truly substantive findings,given that most perform tests to below 1000 cycles58,61–66or perform cyclic tests to low strain rates64. As the fieldprogresses, cycle number in cyclic degradation testingmust also increase if these epidermal electronics are tobe used for the entire lifecycle of the epidermis. Whilethere are still significant impediments to overcome, noninvasive, continuous monitoring has the potential to bringa momentous leap in disease control when combined witha feedback loop-based therapy system.Therapeutic CapabilitiesIn conjunction with a diagnostic sensing and supportsystem, transdermal drug delivery via an epidermaltattoo has the potential for non-invasive therapy as wellas patient-specific regulation of drug delivery67. The skinis constituted by three layers: the waterproof epidermis,which is made up of keratinocytes, melanocytes, merkel andLangerhans cells; the middle layer or dermis, consisting ofhair follicles, sweat and sebaceous glands, nerves, collagen,lymph vessels, and blood vessels; and the last layer, thehypodermis, which consists of the subcutaneous fat layer.The outermost layer of the epidermis, the stratum corneum(SC), acts as an efficacious barrier membrane, limitingdiffusion of large molecules to the dermis67.Multiple methods, both physical and chemical,have been developed to increase diffusion of a targetdrug through the SC, the most prominent of which arethermal enhancement68 (Figure 3A), which uses localizedvasodilation from directly applied heat that can increaseblood flow to a specified area by upwards of 9x, causinga 13x increase in uptake of a drug, such as nicotine69. Theelevated heat can lead to some discomfort for the user andit is also difficult to predict delivery rates, which could leadto the delivery of potentially fatal drug concentrations70. Amore controlled method is iontophoretic delivery (Figure3B)71,72, which uses an applied voltage to transport chargeddrugs across the SC via electrophoresis and electroosmosis,allowing for upwards of a 10x improvement in drugdiffusion over thermal delivery68; however, this methoddoes require large, possibly dangerous voltages to achievePage 9 of 16

Williams NX, Franklin AD. Electronic Tattoos: A Promising Approach toReal-time Theragnostics. J Dermatol & Skin Sci. 2020;2(1):5-16Journal of Dermatology and Skin ScienceFigure 3. Electronic tattoo-based transdermal drug delivery. Delivery methods for diffusion of drugs through the SC layer of the epidermisinclude (A) thermal (using heat to increase diffusion), (B-C) iontophoresis (using an electro-repulsive force), (D) microneedle (puncturingthe SC layer), (E) sonophoresis (using ultrasonic enhancement), and (F) chemical/ encapsulation (coating the desired drug to increasediffusion). (G) A schematic of a complete patch demonstrates the incorporation of a diagnostic sensing component and a therapeuticcomponent that contains one or more of the delivery enhancement strategies. Reprinted with permission from: (A) ref. 80, copyright2014, Nature Nanotechnology; B-C) ref. 68, copyright 2016, Advanced Healthcare Materials; D) ref. 73, copyright 2013, MolecularPharmaceutics; E) ref. 77, copyright 2018, Advanced Drug Delivery Reviews; F) ref. 79, copyright 2015, Colloids Surfaces B: Biointerfaces;G) ref. 46, copyright 2016, Nature Nanotechnology).desirable delivery rates. Another method is microneedledelivery (Figure 3D)73,74, which uses small needles topenetrate through the SC; this poration allows for a desireddrug to bypass the SC and migrate directly to the dermis.Microneedle is frequently used with another methodto further enhance performance. In addition, anothertechnique is sonophoresis75–77 (Figure 3E), which usesultrasound to either heat the skin or increase permittivityPage 10 of 16

Williams NX, Franklin AD. Electronic Tattoos: A Promising Approach toReal-time Theragnostics. J Dermatol & Skin Sci. 2020;2(1):5-16Journal of Dermatology and Skin Sciencevia acoustic cavitation and generally requires a largeultrasonic transducer incompatible with electronic tattoos,and finally chemically enhanced diffusion (Figure 3F)78,which increases permittivity of the desired drug throughencapsulation and thus is difficult to accurately dose intimed increments79. Regardless of the delivery mechanism,transdermal drug delivery requires a sensing componentto take the requisite readings, (as seen in Figure 3G). Ofall these options for skin-based drug delivery that can beelectronically controlled, iontophoresis shows the greatestpromise for electronic tattoos.to the desired location on the skin in a similar manner toplacing a decal-style tattoo or a sticker37,86,87; and directprinting, wherein the device is printed directly onto theepidermis21,88. For a directly printed device, this can beachieved either via a 3D printing technique, such as anextrusion method like FDM, or via a tradition printingtechnique using an inkjet or aerosol jet printer to printthe temporary tattoo directly onto the skin. The transfermethod offers a large array of fabrication approaches thatinclude conventional cleanroom techniques, which allowfor high performing devices through well-establishedprocessing technologies29,41,47,89 (Figure 4A-B); however,these processing technologies are costly and compromisethe needed scalability in cost for bespoke electronicbiomedical tattoos. Whereas printed electronics30,90–92(Figure 4C-D) allow for the custom fabrication of lowcost components93 with challenges related more to theperformance of the printed devices.Iontophoretic drug delivery offers a promising methodfor incorporation into an electronic tattoo system due toits electronic operation, especially when combined withporation of the epidermis using microneedles80,81, andelimination of the discomfort associated with elevatedtemperatures incident to thermal diffusion enhancement82.Iontophoresis enhances drug diffusion through the skinwith an induced electric field. This method functions viaelectrophoresis and electroosmosis, which allows fortransportation of larger molecules ( 500 Da) previouslyblocked by the SC83. The strength of the electric fielddirectly controls diffusion, and thus iontophoreticdelivery can fully control dosage and dosing intervals.However, many embodiments of iontophoretic deliveryrequire non-ideal conditions to enhance drug dosing.Given that the resistivity of human skin is between 1,000to 100,000 Ω84, frequently high voltages are required foreven modest currents required for diffusion enhancement.While numerous publications solely report currents, thepublications that do report voltages use staggeringlyhigh voltages of between 30-90V. These voltages are ofsuch a magnitude to lyse red blood cells84,85 and would bedifficult to implement in standalone electronics; hence,further research is required to decrease the electric fieldstrength required to increase diffusion. One possible routeto accomplish this is by decreasing the electrode gap, thusdecreasing the electrical resistance of the system. Whilethe remaining challenges are significant, motivation forcomplete electronic theragnostic epidermal tattoo systemsis high and thus warrants further research into solutionsfor an on-skin therapeutic drug delivery system.Fabrication and ScalabilityOne main differentiator distinguishing epidermalelectronics from conventional wearables is the applicationmethod. While wearables are generally incorporated intoa rigid electrical device, such as a wristwatch, epidermalelectronics necessitate intimate contact to the skin, andthus must be flexible and stretchable. These limitationsenforce constraints on materials and design selection,both of which limit processing options. There are twosubsets of fabrication: transfer method, where the tattoois fabricated onto a disposable substrate and transferredDirect printing allows for customization to the patient’sneeds and rapid prototyping because there is no delaybetween fabrication and utilization. Direct printing alsoeliminates the potential for errors in transferring, whichplague alternate methods. Yet, direct printing is not yet adrop-in replacement for traditional fabrication methodsand their transfer to skin, as deposition of electronicallyactive materials and inks directly onto biological tissue ofnearly all printed electronics requires caustic or otherwisedamaging post-processing to achieve the desired electricalproperties94. Recently, research interest has grown inthe area of printable electrically conductive inks thatcan be cured at low temperatures95, with a subset thatallow for desired performance at biologically compatibletemperatures96. Yet, even so, the conductivity of theseis often orders of magnitude below that of their bulkequivalents21. The growing interest in in-place-printedelectronics also provides promising developments for theincorporation of more complex components into directprinted epidermal tattoos21,88 (Figure 4E-F).Further development is still required to eliminatebiologically incompatible temperatures and toxic chemicalsand a substantial amount of progress is needed for otherelectronic materials and, ultimately, devices to be printedonto skin. Hence, direct printed electronic tattoos arecurrently limited to only a few uses that focus on electricallyconductive inks for the fabrication of both sensing andsupportive components21,88. Even so, both demonstrationshave used silver nanomaterials, which are known to becytotoxic and, as with other metal nanoparticles, mayalter immune responses97 and might thus eventuallycause cancer98,99. While silver nanowires are less cytotoxicthan silver nanoparticles, frequent use still remains aconcern, especially with the incorporation of heating andmicroneedles to increase penetration into the dermis as thereduction of elemental silver (Ag0) to ionic silver (Ag ) is atPage 11 of 16

Williams NX, Franklin AD. Electronic Tattoos: A Promising Approach toReal-time Theragnostics. J Dermatol & Skin Sci. 2020;2(1):5-16Journal of Dermatology and Skin ScienceFigure 4. Fabrication techniques for electronic tattoos. The transfer method involves fabrication onto a disposable substrate either via(A-B) traditional, cleanroom fabrication technique schematic process flow or (C-D) via a printing method where the material is depositedin a solution form onto the transfer substrate. (E-F) The direct printing technique involves deposition of the electrically active inks directlyonto the desired biological tissue. Reprinted with permission from: (A) ref. 90, copyright 2018, Small; B) ref. 89, copyright 2017, ACS Nano;C) ref. 92, copyright 2018, ACS Applied Materials & Interfaces; D) ref. 91, copyright 2018, Nature; E) ref. 88, copyright 2018, AdvancedMaterials; F) ref. 21, copyright 2019, Nanoscale).Page 12 of 16

Williams NX, Franklin AD. Electronic Tattoos: A Promising Approach toReal-time Theragnostics. J Dermatol & Skin Sci. 2020;2(1):5-16Journal of Dermatology and Skin Scienceleast a contributing factor to its toxicity and heat

Electronic tattoos are nonpermanent electrical devices or systems placed in intimate contact with the skin and intended for relatively short-term use (upwards of 1-2 weeks). Their name is derived from their similarity to temporary, decal-style tattoos rather than an ink embedded into the dermis to change local pigmentation. While there