Thin-film electronics on active substrates: review of materials, technologies and applications

The electrical properties of electronics on unconventional substrates are less relevant than for standard electronics, but are still of paramount importance for the functionality of a system. As in conventional electronics, they include the mobility [63], speed [64], and operation voltages [65] of transistors, the complexity [25], gain [66], and noise performance [67] of integrated circuits [68], the sensitivity and accuracy of sensors [69], and the power consumption [3] of the thin-film systems itself. Additionally, the potential electrical functionality of the substrate has to be taken into account. These include stretchable and bendable interconnections [70], the dielectric properties [71] and transparency [53] of the mechanical support, and sometimes the substrate even is a part of the thin-film device itself e.g. by acting as the gate dielectric of thin-film transistors (TFTs) [72].

With the possibility to explore new fields of applications, unachievable for standard rigid electronics, the mechanical properties of these systems are a key factor. First of all, robustness [73], during fabrication and under normal usage conditions, guarantees system stability and proper functionality. This implies resistance to a variation of the environmental conditions (temperature and humidity) and mechanical loads [74]. Intrinsic mechanical properties, such as Young's modulus, strength, resilience, ductility, and rigidity, provide indications on which application could be tackled. As a result, the minimum bending radius [75], maximum stretching strain [76] and conformability [56] to the target surface are significant.

Similarly to mechanical properties, these features are essential for unconventional systems. Chemical resistance to both reagents and harsh environments [58] are crucial, for fabrication protocols and final usage purposes. Material composition [77], in combination with biocompatibility [78], are of paramount importance, while aiming for green and dissolvable devices [79], implantable systems and edible electronics [80, 81].

Polymers define a broad range of materials. Their mechanical flexibility is attractive for the possibility to have stable thin-film electronics performance on non-flat surfaces. Imperceptible and conformable electronics are achieved by ultra-thin, lightweight, and transparent polymers [53, 82–84]. Stimuli-responsive materials like hydrogels and shape memory polymers or stretchable polymers, such as elastomers, can further improve the electronic's mechanical adaption [46, 85–90]. Polymeric substrates can be electrically active due to percolation pathways of particles in the elastomeric matrix [91], or a conductive filler dispersion such as carbon black [92] or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) [93].

Degradable, bioresorbable, and biocompatible active polymers have been pointed out for green, recyclable, and flexible electronics [54, 55, 79, 88, 94]. All these properties are fundamental for wearable devices [92, 95–97], on-skin monitoring system [46, 88, 91, 98], and soft robotic applications [23, 99, 100].

Polyimide (PI) is the most common employed substrate for flexible thin-film electronics with a thickness ranging from ≈1.5 µm to ≈50 µm [23, 48, 52, 68, 83, 92, 95, 96, 98, 101–113]. PI foils are commercially available [95, 106] alternatively membranes can be spin-coated on a temporary carrier [48, 83, 110]. Its thermal stability is characterized by a low thermal expansion coefficient of 3.4 ppm ^∘C⁻¹ and a high glass transition of 360 ^∘C⁻¹ temperature [107] allowing processing temperatures up to ≈300 ^∘C⁻¹ [83]. These properties and their chemical stability make it suitable for thin-film fabrication techniques (sections 3.1 and 3.2) involving standard photolithography processes, lift-off, and wet-etching [104, 105, 108]. Its natural flexibility, as shown in figure 2(a) ensures functionality under variable bending conditions: for 50 µm thick PI a minimum bending radius of 1.7 mm was achieved [103]. This can be further reduced to 125 µm [101] through neutral strain plane designs, as described in section 4.6. Despite ultra-thin PI layer showing a transparency of 90% [83], PI is generally characterized by an amber colour, which makes other polymers more suitable for transparent electronics. These include parylene [53, 82, 84, 99, 114, 115], polyethylene terephthalate (PET) [41, 98, 116–120], polyethylene naphthalate (PEN) [92, 121], and polymethyl methacrylate (PMMA) [29, 122].

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Parylene is a flexible and transparent polymer acting as a substrate or as insulating layer (figure 2(b)). Tens of micrometer-thick substrates [82, 114], as well as ultra-thin membranes around 1 µm are obtained by evaporation process [53, 84, 99, 115]. Due to this low thickness, thin-film electronics fabrication must be performed while depositing the polymer on a rigid carrier covered by a sacrificial layer [53, 84, 115]. Thin-film electronics fabricated on parylene is bent down to 50 µm as shown in figure 2(c). The minimum bending radius decreased by decreasing the strain induced by bending a 1 µm-thick substrate [53]. Similarly, PEN foils (generally 50 µm thick), as well as, PET foils with a thickness ranging from 1.4 µm [118] up to 175 µm [116] are commercially available. Their flexibility is beneficial for the conformability of epidermal electronics [98, 118] (figure 2(d)) or smart textile applications [41, 92].

PMMA is a transparent and flexible polymer that can act as substrate [29, 122] and dielectric layer [120, 123, 124]. Thin-film transistors on micrometer-thick PMMA ($\lt$2 µm) showed stable electronics performance when cycling bending test at a minimum bending radius of 1 mm was performed [122]. As with other solution-based polymers, the electronics fabrication can be carried out by spin coating the PMMA on rigid support. Free-standing devices on PMMA substrate are obtained through transfer printing process as described in section 3.6. Moreover, PMMA itself acts as sacrificial layer enabling free-standing electronics on other polymeric substrates thanks to its solubility in organic solvents [125–127].

Rubbery elastomers such as polydimethylsiloxane (PDMS) [49, 51, 83, 85, 86, 91, 99, 100, 110, 128–139] and Ecoflex [46, 88, 137, 140–142] are mainly attractive for their stretchability that is missing in other plastic materials. These features are particularly beneficial for wearable and epidermal patches [46, 88, 91, 142]. PDMS is characterized by a low Young's modulus of ≈2 MPa [49, 134]. The stretchability of PDMS, acting as active substrate, depends on both the elastomer deposition and design. Casting deposited PDMS was strained up to 5% [49, 86], but a maximum applied strain of 20% was achieved when an 800 µm thick pillars' mould was exploited [133]. This geometry allowed a twisting of the substrate up to 180^∘. Spin-coated PDMS swells during the curing process due to its high thermal expansion coefficient of $3 \times 10^{-4}$ K⁻¹ [128]. Constrained deformation of PDMS is achieved by depositing it on material with lower thermal expansion coefficient enabling the fabrication of thermal PDMS soft actuators [99, 110] or electronics on non-flat surfaces for skin applications [51]. Other strategies to take advantage of the PDMS stretchability involve engineered strain distribution and are described in section 4.6. PDMS is also transparent [49, 51, 83]. Ecoflex is transparent, commercially available, natural, and biodegradable [88, 140] and its low Young's modulus of 0.69 Pa ensures a stretchability $\gt$100% [46].

Biodegradable and wast-free flexible electronics can be achieved by using cellulose-based substrates that also bring the advantages of being cheap and largely available [79, 94]. Depending on the production process, it can exhibit chemical stability and a glass transition temperature $\gt$180 ^∘C suitable for direct fabrication [79] or transfer printing processes [94]. Less than 5% of variation on the characteristic curve of electronics circuits is observed when cellulose is bent down to 2 mm [79]. Another example of a bio-substrate is keratin extracted from human hair [143]. Uniform layers of keratin were spin-coated on a Si carrier acting as substrate or gate dielectric. A bending test at a radius of 20 mm was performed to prove the flexibility. Additionally, a transient behaviour was showed when exposed to alkine-solution. Polypropylene based paper offers another approach for eco-friendly electronics [144]. Its thermal and chemical stability is suitable for direct thin-film fabrication while the paper is laminated on a rigid support. After releasing, the bending test was performed while the electronics functionality was preserved proving the potential paper suitability as flexible eco-substrate. Thin-film electronics on parylene membrane transferred on 100 µm thick polypropylene foil showed improved mechanical properties [53, 84]. The poor adhesion between the two polymers reduced the strain on the electronics allowing its functionality under several crumbling of the substrate [53] or an increase of the maximum stretchability [84]. Polypropylene is also employed in form of natural and lightweight fibers suitable for smart textile [145, 146].

Shape memory materials are a unique class of materials that presents the ability to change the shape upon the application of external stimuli (e.g. temperature [147–149], magnetic field [150, 151], and chemical conditions [152, 153]). In a macroscopic point of view [154], this effect has been verified in diverse materials, such as metal alloys [147, 155, 156], ceramics [157, 158], and polymers [50, 90, 159–167]. They have been extensively used in applications ranging from medical [90, 154, 168–171] to aerospace engineering [147, 172, 173]. Recently, one of the most promising applications is the use of shape memory polymers with flexible electronics [159–164, 166] and sensors [50], in which they are mainly used as substrates.

Many shape memory polymers have been used as substrates [174], such as cross-linked shape memory polyacrylate [175], which has been used as a substrate for light-emitting diodes when combined with silver nanowires or single-walled carbon nanotubes; polycaprolactone (PCL) [176], which was used to create a structure responsive to heat; and also poly(tert-butyacrylate) (PTBA) [167], which has been applied in active tactile displays. However, thiol-ene/acrylate [177] is one of the most used shape memory polymers as substrate for thin-film and commercial electronics [90, 163, 165], such as electrodes [160], OLEDS [161], capacitors [162], sensors [50], and TFTs [163, 164]. Moreover, robust and reliable process can be used to fabricate electronics onto this material. For example, pentacene organic thin-film transistors (OTFTs) were fully structured by photolithography in an array distribution pattern [163] (figure 3(a)). The OTFTs' performance was similar to the ones realized on rigid substrates fabricated with shadow mask processes, even after going through 100 bending cycles. Similarly, OTFTs can also be fabricated onto the thiol-ene/acrylate using low-temperature solution-processing, enabling high-performance OTFTs [164]. 3D e-whiskers were developed using thiol-ene/acrylates as substrate to receive multi-stimuli at the same time (figure 3(b)) [50]. Gold strain-gauges were photolithographed on the substrate and laser cutting was used for the outer perimeter of the whiskers (figures 3(c) and (d)). The device was able to map delicate surfaces, such as fingerprints, with results similar to profilometers (figure 3(e)). Additionally, thiol-ene/acrylate can be used as an insulator [162].

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During the last decade, the interest in degradable electronics has been growing to address the exigency in reducing electronics waste, usually involving toxic, long-decomposable, and rare materials [140, 143, 178–180]. Research focused on bioresorbable and biodegradable materials to meet the requirements for smart medical implants (section 5.1.2), avoiding secondary surgery procedure, tissues inflammation, and enhancing the post-operative recovery [54, 55, 60, 115, 181–184], and favouring a sustainable approach for electronics fabrication. In this view, substrate materials and strategies for so-called transient electronics that can be dissolved are presented in the following.

Polyvinyl alcohol (PVA) is a water-soluble transparent polymer that can be used as substrate for transient electronics [185–187]. The PVA dissolution time in deionized water is around 30 min [186, 187], but it depends on many parameters such as thickness of the layer, water temperature, and post-deposition treatment [126]. Liquid metal patterned on PVA was completely recollected after polymer dissolution, proving its suitable for fully recyclable systems [185]. Shadow mask patterning [126] or transfer printing process [186, 187], as described in sections 3.3 and 3.2, are generally preferred for PVA.

Poly-anhydrides constitute a class of polymers suitable for transient electronics [183, 188–190]. Acid compounds by-products following their dissolution in water enhance the degradation of no-transient metals [188]. Their dissolution time depends on temperature, pH, and chemical composition [189, 190]. A 120 µm thick layer showed a dissolution rate of 1.3 µm d⁻¹ [183] suggesting the suitability of poly-anhydrides as encapsulation layer for electronics and biomedical implants [183, 190].

Poly lactic-co-glycolic acid (PLGA) is also used as transient substrate [54, 60, 125, 183, 190]. PLGA dissolves in phosphate buffer solution (PBS) at physiological temperature (37 ^∘C) over weeks (researchers reported a period from 4 to 8 weeks) and its biocompatibility makes it suitable for long-term health monitoring [54, 60, 183].

Naturally derived materials, such as cellulose [79] or silk [55, 182], are also employed for biodegradable and transient electronics as well as water-soluble and edible rice paper [125].

The electronics shut-down can be achieved by stimuli-responsive materials such as cyclic poly(phthalaldehyde) (cPPA) where UV-exposure or temperature triggers an acid molecule release leading to degradation [191, 192]. Accordingly, direct electronics fabrication involving UV-photolithography on this substrate is avoided and it is replaced with transfer printing [191] or stencil mask [192]. Devices for food packaging control are envisioned applications of this approach [193, 194].

In recent years, there has been a significant interest in wearable electronics and this has lead to the integration of electronic functionality into everyday textiles [195]. Textiles have been preferred for wearable application due to it is familiarity and ability to comfortably conform onto the human body. Most of the flexible structures that are utilised in flexible electronics are unable to conform onto body shapes because they can only deform along one single dimension and often fail under twisting or other severe deformations. For this reason, effort has been devoted to employ textile fabrics and yarns as substrates when developing electronic devices [113, 196–212]. In general textile materials are washable, permeable to gases or liquids, able to withstand even severe mechanical deformations such as folding or sheering and in some cases they are also stretchable. These features make textiles a desirable substrate for thin-film electronics and highly suitable for smart textile and wearable applications (section 5.5).

Researchers have utilised knitted or woven textile fabrics as substrates [197, 199–201, 204, 209, 211–213]. These fabrics were made from both natural fibers and synthetic fibers such as cotton [209, 211], polyester [201, 209], meta-aramid [201], nylon [199–201, 212, 213], glass fiber [197], kermel [209], polyurethane [212], elastene [213], polypropylene [145, 146], and silk [210]. The devices fabricated on the textile include electrodes [199, 201, 212], resistors [200], transistors [204], sensors [213], solar cells [197], piezoelectric fabrics [209] and thermoelectric nanogenerators [211].

Textile threads and fibres were also used as substrates to fabricate thin film electronic devices [113, 196, 198, 202, 203, 205–208, 214]. Textile fibres are the raw materials for manufacturing textile fabrics. In some cases, electronic devices were built directly on the textile fibres and yarns [113, 196, 205–207], whereas in others these fibres were utilised within a textile structure to create an electronic device [198, 202, 203, 207, 208, 214]. The most commonly used fibre is nylon [113, 196, 202, 214] but researchers have also utilised Kevlar multifilament [203], PET fibres [198], silk [207], glass fibre [196], acrylic [205] and linen [206]. Devices fabricated using these functional fibres include sensors [113, 198, 207], transistors [196, 202, 203, 205, 206] and piezoelectric generators [208].

Table 1. Thin-film devices and their performances for the active substrates described in section 2.1. Each row underlines the best example, among the reviewed ones, for a specific property.

Substrate material and thickness	Devices	Electronics performance	Mechanical properties	Features	Reference
Parylene 1 µm	TFT	µ = 11.3 cm2 V−1 s−1 V$_{th}$ = 0.4 V SS = 180 mV dec−1 $I_\mathrm{on}/I_\mathrm{off}\gt 10^{7}$	Max. strain: 210%	Biocompatible	[84]
PMDS 80 µm	TFT, logic, circuits and rectifiers	µ = 13.7 cm2 V−1 s−1 V$_{th}$ = 0.1 V SS = 130 mV dec−1 $I_\mathrm{on}/I_\mathrm{off} = ~10^{7}$	Bending radius: 13 µm	Biocompatible and imperceptible	[51]
Cellulose 800 µm	TFT, logic and circuits	µ = 0.12 cm2 V−1 s−1 V$_{th}$ = 5.75 V $I_\mathrm{on}/I_\mathrm{off}\gt 10^{4}$	Bending radius: 2 mm	Green and biocompatible	[79]
PLGA 30 µm	TFTs and array	µ = 200 cm2 V−1 s−1 V$_{th}$ = 1 V $I_\mathrm{on}/I_\mathrm{off} = ~10^{8}$	Bending radius: 1 mm	Transient and bioresorbable	[54]
Polypropylene 30 µm	Capacitive + resistive touch display and LED display	Rise and fall time: 1.4 ms, emission peak at 500 nm	Bending radius: 10 mm and repeated torsion: $\gt$1000 cycles	Transparent	[146]
Silk worm fiber	Temperature and pressure sensors	Temperature sensitivity: $1.2 \times 10^{-2}\ ^{\circ}$C−1, pressure sensitivity: 0.136 kPa−1	Bending angle: 0∘–360∘	Biocompatible and washable (3 × washing machine with detergent)	[207]
Gold microfiber 100 µm	TFTs	V$_{th}$ = −1.01 V $I_\mathrm{on}/I_\mathrm{off}\gt 10^{5}$	Bending radius: 2.0 mm	Washable (beaker with detergent)	[142]
Thiolene/Acrylate 50 µm	OTFT	V$_{th}$ = −7 V µ = 0.018 cm2 V−1 s−1	Bending radius: 8.5 mm (100 cycles)	Recovareble after deformation	[163]

When developing thin-film structures on textile substrates it is vital to consider the impact of bending, sheer and twisting of the textile structure on the performance of the fabricated device. Furthermore, the literature illustrates that the surface roughness of textile yarns and fabrics have also had an impact on the device performance [113, 196, 201]. To reduce the influence of surface roughness researchers have employed techniques such as ultraviolet-ozone treatment [201] and coating or lamination of the textile prior to creating devices [199, 200]. Hence, surface roughness of the textile substrate and the ability of the rest of the device to withstand multi axial mechanical deformations remain as essential factors to consider when developing thin-film devices on textiles substrates.

Thin-film structures were also fabricated on cylindrical substrates such as metal wires [47, 142, 215], glass fibers [216], and optical fibers [217–219]. These devices were mainly utilised to create transistors for electronic textile applications [142, 215, 217–219]. The high conformability of wires enables them to be used as substrates in smart textile applications. Therefore, the feasibility of synthesising devices on copper [215], gold [142] and aluminum [47] wires were evaluated. On one occasion the wire was cleaned and heated to form an oxide coating [215], whereas in others they were coated in a PDMS or P3HT solution [47, 142]. For the glass fibers, metals like chromium, copper, and gold were evaporated all around the fiber [216]. The ability of these substrates to withstand harsh environments makes them suitable for applications such as structural health monitoring of construction materials. In the case of optical fibers, metals such as chromium and AlO_x were directly deposited on the fiber substrate to fabricate devices [217]. Optical fibers have been utilised for smart textiles but their limited flexibility makes them unappealing for these applications.

More exotic materials such as natural wax [220] and ultrathin insulating poly(vinyl formal) (PVF) [221], were also utilised as substrates for creating flexible electronics. The electronic devices created using these exotic materials include transistors [221] and NFC circuits [220]. In addition, the device created on PVF was able to withstand a bending radius of 0.7 µm.

Semiconductors in thin-film electronics are employed as active layers, defining the device technology and carrier transport efficiency. Traditional semiconductors such as silicon (Si) are well-known for their high mobility and for being mechanically rigid and brittle. To employ these materials in thin-film electronics, silicon in the form of amorphous-Si (a-Si) [129, 222] or Si nanomembranes (Si NMs) [54, 55, 60, 125, 191, 192] have been used. The small mobility (less than 1 cm² V⁻¹ s⁻¹) that can be achieved with a-Si TFTs represents a limitation, while Si NMs represent a better alternative because of their higher mobility ($\gt$500 cm² V⁻¹ s⁻¹) [223]. Moreover, Si NMs were largely used in transient electronics due to their fast dissolution time [125, 192]. Poly-crystalline Si shows a mobility between 50 and 100 cm² V⁻¹ s⁻¹ [224, 225]. Despite the better performance of Si NMs and poly-crystalline, their fabrication processes are expensive and time-consuming [226].

Organic semiconductors have the advantages of easy and low-cost manufacturing, mainly based on solution processes (section 3.2), and room temperature processability suitable for direct fabrication on plastic, fabrics, and transient substrates. They are mostly p-type semiconductors such as pentacene [102, 123, 136, 163, 204, 227], carbon nanotubes (CNTs) [98, 186, 206, 218, 219, 228, 229], polymeric semiconductors [79, 116, 118, 143, 230, 231] among which the most reported is poly(3-hexylthiophene) (P3HT) [91, 93, 142, 204, 206, 219, 232], or naturally derive semiconductors such as indanthrene yellow G, indanthrene brilliant orange RF, and perylene diimide [140, 179]. They are lightweight and flexible due to their chemical nature, undergoing large strain up to 50% when mixed with stretchable materials like PDMS [91] or Styrene-ethylene-butylene-styrene (SEBS) [230]. Organic naturally derived semiconductors also allowed the fabrication of edible OTFT [140, 179]. Current limitations arise from the less developed n-type semiconductors and the small achievable mobility, typically $\lt$10 cm² V⁻¹ s⁻¹.

Metal oxide semiconductors have been used in the last decades due to their promising properties for thin-film electronics such as the low temperature processability ensuring high quality deposition, high mobility, optical transparency, mechanical stability, and the possibility to achieve large-scale fabrication [226, 233]. They are mostly n-type, and among them, transparent amorphous indium-gallium-zinc-oxide (a-IGZO) is the most used one [51–53, 68, 82, 84, 85, 101, 103–107, 109, 128, 131, 133–135, 137, 187, 188, 234–237] showing a mobility $\gt$10 cm² V⁻¹ s⁻¹ [238], large-area processability, and low deposition temperature [20]. Other n-type amorphous metal oxide are zinc-oxide (ZnO) [49, 120, 239], zinc-tin-oxide (ZTO) [112], indium-oxide (In₂O₃) [131, 164], while p-type amorphous metal oxide are nickel oxide (NiO) [51] and tin-oxide (SnO_x ) [72].

Insulating materials in thin-film electronics are employed as passivation or buffer layers, and as dieletrics. The dielectric constant and the thickness are the fundamental parameters. Good insulating properties ensure a proper isolation of components and circuits, preventing leakage current, and voltage breakdown.

Insulating materials are silicon oxide (SiO_x ) [49, 54, 55, 60, 83, 88, 125, 127, 132, 186, 187, 236, 237], aluminum oxide (Al₂O₃) [51–53, 68, 79, 84, 101, 103, 105–109, 112, 118, 128, 131, 133, 135, 234, 235, 240], hafnium oxide (HfO) [3, 165, 228], magnesium oxide (MgO) [55, 188], silicon nitride (SiN_x ) [54, 88, 129, 131, 186, 187, 236] or polymers such as parylene [86, 112, 114, 136, 163], polyimide [241, 242], SU8 [49], hydrogels, [116] and ion-gels [91, 93]. Moreover, organic dielectrics are glucose, lactose, or nucleobase [179].

SiO₂ is a bioresorbable insulating material employed as electronics dielectric layer [49, 55, 60, 83, 125, 127, 236] or passivation/encapsulation layer [54, 55, 83, 88, 236]. It is deposited by plasma-enhanced chemical vapor deposition (PECVD) (section 3.1). Oxide layers ranging from 50 nm to 900 nm are water soluble [55, 125, 127], and their solubility is also proved on saline [29] and PBS [54, 60, 125] solutions. The large SiO₂ layer thickness is required when it is employed as a gate dielectric layer because of its low dielectric constant, around 4. For this reason, high-k dielectric materials [243] are preferred for thin-film electronics e.g. Al₂O₃ exhibiting a dielectric constant of ≈8.5 [79] or HfO₂ acting as gate dielectric with a thickness of 20 nm [228].

MgO is used as gate dielectric for transient thin-film electronics [191], and it can also act as passivation layer to tune the dissolution time of the transient electronics according to its thickness [55]. SiN_x can be deposited as a single layer [129] or in sandwich structures with SiO₂ [54, 186, 187] acting as passivation layer, gas barrier layer [83], or humidity barrier [236].

Although polymeric dielectrics have lower dielectric constant than SiO${_x}$ ($\epsilon_\mathrm{polyimide}$ = 3.24 [242], $\epsilon_\mathrm{parylene}$ = 4 [244]), they are suitable as substrate for their stretchability and bendability. Parylene substrate hosting TFTs with parylene dielectric-layers can undergo up to 10 000 bending cycles [114], while hydrogel dielectric based TFTs can withstand a maximum strain of 55% [116]. Additionally, shape memory polymers can also be used as insulators providing two different dielectric constants and resistivity depending on the temperature. For example, thiol-ene/acrylate has a dielectric constant of 5.26 ± 0.11 and 6.38 ± 0.11, below and above the transition temperature, respectively [162].

Sandwich insulating structures are fabricated for improved dielectric performance due to the combination of different dielectric materials [231, 245]. Both polymers and organic dielectrics are deposited in the form of bilayer dielectric structure with Al₂O₃ [102, 112, 134, 140]. OTFTs based on nucleobased dielectric layers showed larger capacitance per unit of area (from 5.6 nF cm⁻² to 23.4 nF cm⁻²) when forming a hybrid organic/inorganic with Al₂O₃ [140].

Most of the employed conductor materials for thin-film electronics are metals. Copper (Cu) [51, 133], gold (Au) [129, 131, 136, 234], and aluminum (Al) [86, 112, 239] are extensively used because of their high electrical conductivity of $59.8 \times 10^{6}$ S m⁻¹, $42.6 \times 10^{6}$ S m⁻¹, and $37.7 \times 10^{6}$ S m⁻¹ respectively [246]. Chromium (Cr) [234, 235], and titanium (Ti) [51, 231, 241] have a low electrical conductivity, are employed as adhesive layers to improve the adhesion of other metals like Au [132, 231, 237, 241], platinum (Pt) [132, 241], palladium (Pd) [186]. However, Cr is also less ductile, showing a smaller rupture strain level ($\lt$0.5%) compared to others, such as Cu, limiting the minimum achievable bending radius [103].

Molybdenum (Mo) [54, 83, 186, 187, 236] and magnesium (Mg) [55, 60, 88, 125] are commonly employed for their biodegradability and biocompatibility. Moreover, Mg is also water soluble and suitable for bioresorbable implants, while Mo can degrade in acid environment as zinc (Zn) or iron (Fe) [191].

Fully transparent electronics are fabricated through transparent conductive oxide such as indium-tin-oxide (ITO) [49, 51, 82, 107, 116]. However, it was shown that they easily experience fractures and delamination [53].

Metals can be deposited through vacuum-based technologies (section 3.1), some can also be solution processed (section 3.2).

Additionally, so-called liquid metals, consisting of metals and metals alloys that are liquid at room temperature, have been used for compliant and microfluidic electronics. Within this class, galinstan (GaInSn) [139, 185, 247], and eutectic gallium-indium (EGaIn) [139, 247, 248] are the most promised candidates because of their electrical conductivity of $3.40 \times 10^{6}$ S m⁻¹ and low melting point equals to 19 ^∘C⁻¹ and 15.5 ^∘C⁻¹ respectively.

Finally, carbon-based conductors are also employed in thin-film electronics in the form of CNTs [207, 230] or as conductive fillers in polymeric matrix [92].

The suitability of thin-film electronics for on-skin applications relies on the intrinsic properties of active substrates. The employment of stretchable and flexible materials has allowed systems with proper adhesion and conformability to the epidermal roughness [46, 51, 84, 88, 91, 98, 126, 345]. Transparency and lightweight guarantee imperceptible and unobtrusive systems [51, 53, 82, 126, 141]. Hypoallergenic patches avoiding skin inflammations benefit from biocompatible materials ensuring safe electronics operations [30, 77, 88]. Additionally, they can biodegrade, allowing the development of disposable medical systems. The applications can consist on diagnostic systems [98, 126, 346, 347], wound healing process [348] or skin conditioning sensors [46, 91].

A multi-functional device for biosignal monitoring consisting of temperature, UV, electrocardiogram (ECG) sensors, and printed accelerometers, for motion sensing, was fabricated on PET and PI substrates [98]. A kirigami-like design of the substrate, as described in section 4.7, was employed to prevent stress and failure of the electronics under strain when applied on a human chest and tested during different motoric activities. The temperature and UV sensing process relied on CNT TFTs, showing a constant outcome when no temperature and light variations occurred while running/rest tests proved the ECG sensor and the accelerometer's detection capability. An EMG sensor was also fabricated on a PVA substrate because of its capability to ensure a proper adhesion on-skin [126]. Thermal treatment of PVA allowed the customization of its swelling rate due to water or moisture adsorption as described in section 2.1.3, ensuring mechanical conformability and sensor functionality. The employment of elastomers prevents electronics failures, even while a mechanical strain is applied. In this regards, organic TFTs pressure and strain sensors were fabricated by dispersing conductive and semiconducting materials within a stretchable PDMS matrix and tested as an artificial skin on a robotic hand [91]. The system was used for sign language interpretation and was characterized by a stable gauge factor around 33, while a strain from 10% to 50% was applied. A printed pressure sensor embedded on an ecoflex-hydrogel bilayer was also proposed to guarantee the electronics functionality under mechanical stress [46]. The low modulus of elasticity of the hybrid substrate, which is around 0.005 Pa, ensured a proper sensor response when applying on fingers undergoing folding/flatten cycles.

Smart epidermal systems also required the development of a wireless powering system and signal processing to be non-invasive. A Bluetooth transmission system consisting of a flexible and biodegradable sensor was proposed [88]. Despite the device being portable, it resulted in a tethered system because of the connection with the rigid electronics board. The suitability of a thin-film rectifier as wireless energy transmission circuits based on inductive coupling was proved on both parylene [84] and PDMS [51]. Although the electronics miniaturization ensured the imperceptibility of the circuit, this limits the maximum output power.

Current challenges remain the integration on epidermal patches of an efficiency powering system and the energy transmission over long distances.

Researches on biocompatible materials have encouraged the fabrication of in-body devices as a promising approach for non-invasive medical procedures [60, 80, 181, 182, 184, 349] and biosignals mapping [54, 115, 132, 183, 223, 251, 350].

Materials for functionalized implants must be chemically stable when immersed in biofluids ensuring reliable electronics performance and providing sufficient mechanical adhesion on the target organ without compression damages. In this regard, shape-memory materials (section 2.1.2) have been shown their suitability because of the possibility to control their stiffness. For example, thiol-ene/acrylate is characterized by a reduction of the glass transition temperature (from 70 ^∘C to 37 ^∘C) when exposed to a physiological environment that lessens its Young's modulus [118]. Hence, the softening of the polymer after the implantation has been explored to develop cortical probes for in-vivo recording of the neural activity [350]. A flexible polyimide/PDMS substrate that can be wrapped around blood vessels by air-pressure control, as described in figure 6, has also been proposed to achieve proper implant adherence [132]. Here, thin-film temperature and strain sensors were fabricated showing a reliable detection of blood vessels pulses compared with the conventional measuring system.

Vital signals monitoring also demand a data acquisition and transmission system. In-vitro experiments on a Swiss-roll nerve cuff (described in section 4.7) have reported the performance of IGZO transistors and circuits in detecting ionic liquid when neural stem cells were growth within the micro-tubular structure [234]. It was also proved the monitoring of cardiac cells proliferation in a tissue-repairing patch due to interaction between the cells in a nanofibre scaffold coating a Au electrodes matrix [349]. Here, the electrodes were used for signals detection from the cells and their stimulation, but also to activate the release of drugs hosted in a porous hydrogel membrane within the matrix, as shown in figure 12(a). The charged structure of the hydrogel allowed the on-demand release of the drug molecules stored in the hydrogel matrix thanks to its swelling during immersion in the solution. To achieve a transmission system in in-vivo and untethered system, as well as the control of the implants functionalities, electronics, and biological parameters, have been accounted for a trade-off between antenna dimensions and power efficiency. As an example, a miniaturized helical antenna was optimized to provide good efficiency in industry-scientific frequency and accounted for signal penetration between human tissue [251].

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Transient materials for implantable systems have gained attention for the possibility to preserve stable functionality before dissolution in the surrounding environment, leaving no harmful residuals [54, 55, 60, 115, 181–184]. Despite surgical processes are still required for the implantation, the implanted devices can overcome the limitations of standard therapies enhancing the effect of the treatment, and their bioresorbability prevents possible injuries from a second extraction surgery [60]. Here, a wireless receiver antenna (figure 12(b)) interconnected with a Mg electrode-cuff was encapsulated in a 30 µm thick PLGA substrate and used for in-vivo electrical stimulation of nerve and muscle. The device resulted in a quicker recovery when compared to standard therapies, and the resorption of the system within 2 months was ensured by both the PLGA dissolution and electronics encapsulation. Due to the dependence of the transient time on the PLGA thickness, it is possible to customize the device's working time [54]. Here, a multiplexed array of Si NMs metal oxide semiconductor field effect transistor (MOSFET) (from 64 to 128) was also fabricated on the same substrate for recording cerebral cortex activity. In-vitro experiments demonstrated the dependence of dissolution time on doping and thickness of the NMs allowing a proper design of the system according to signal monitoring duration. The performance of the device was validated by comparison with clinical equipment and carried out over one month, suggesting the importance of wireless data communication system. A wireless and bioresorbable heating system based was also fabricated on silk natural fiber [182] because of its proved biocompatibility [55]. Additionally, wireless data circuits must be conformable to guarantee the signal acquisition over prolonged periods. As it is shown in figure 12(c), an elastic and bioresorbale polyurethane encapsulation combined with serpentine Mo electrodes allowed the minimization of mechanical stress on the electronics during in-vivo tests that could cause device failure [181]. This design, as described in section 4.5, reduced the electronics strain and ensured a stable operation up to a maximum uniaxial strain of 20% and a dissolution time of 50 d.

The rapid advances in materials and system functionality have guided the exploration of digestible and edible electronics [36]. This new class of smart implants envisions the tracking of biomedical signs and disorders by inserting electronic systems in the gastrointestinal tract, paving an innovative route in the field of health monitoring and telemedicine. Although initial prototypes were suitable only for swallowing (endoscopy capsules [351] and smart pills [352, 353]), the ongoing trend in this field is to design and realize completely digestible systems, safe for both the human body and the environment, with no need of recollection [36, 352]. Fully functional systems require the combination of sensing or actuation features (i.e. temperature, pH, gas, pump reservoir), with powering (i.e. batteries) and data transmission unit (i.e. antennas), reducing the risk of rejection [354]. To achieve these requirements, a step forward is constituted by the use of biocompatible, transient, and natural food-based materials in the device stack [355]. Passive components, such as capacitors, antennas, conductors [356], as well as active devices, like field-effect or electrolyte-gated transistors [81, 140, 357, 358], filters [359], and logic circuits [358], have been presented.

Soft robots have been gaining attention in the last few years for being composed of flexible components that exhibit many capabilities, including high stretchability, the ability to conform to irregular geometries or surroundings and interact smoothly with biological tissues [262, 370–373].

Substrates can be used as components of sensors or actuators in soft robots. For sensing applications, they are integrated on the soft robot's external surface for proprioception, temperature, tactile, or force sensing [50, 374]. For actuating, they can be used to hold and guide the moving components [99] or they can be the actuators themselves (section 6).

Electrically conductive silicone filled with carbon black was used to realize kirigami-inspired soft strain sensors, enabling a feedback of the 3D configuration of the structure [375] (figure 13(a)). To measure objects slippage, PEN was used to fabricate a ferroelectric polymer-based soft sensor considering parallel and perpendicular forces (figure 13(b)) [121]. A PDMS layer was employed to form a substrate for a T-type and a K-type thermocouples that were integrated on a soft micro-finger, conferring tactile sensing abilities to the soft robot [376]. Figure 13(c) shows a close-up view of the T-type temperature sensor. Additionally, some applications can utilize substrates inside the robot structure rather than on the external surface, as demonstrated by Xie et al [100]. Here, a flexible piezoelectric PVDF thin film combined with jamming layers was developed to provide proprioceptive sensing (figures 13(d) and (e)).

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As part of actuators, PDMS was combined with PI and Au to realize a bio-inspired ultra-thin controllable device that could simulate the movements of a chameleon's tongue to hunt moving insects or mimic the plant's vine in winding around a tiny pole [110]. A combination of Au electric traces in a liquid crystal elastomer doped with carbon black (LCE-CB) with a PI substrate layer was used to selectively control different parts of the substrate (figure 13(f)), mimicking the locomotion patterns of inchworms [48]. In another example, the shape morphing was controlled through a combination of PDMS with PEDOT:PSS with hygroscopic properties to form a single composite material that could change through specific environmental humidity or the application of electrical current [130]. Figures 13(g)–(h) show the robot in non actuated and actuated states, respectively.

Miniaturized systems have been gained attention for the envisioned functionalities they could achieve in medical technologies [377], such as drug delivery, or non-invasive surgery, and for environment sensing and pollutants degradation in energy and green application [378]. Despite smart micro-machines, such as self-propelled micro-engines [379, 380], cargo loading/transport platform [381, 382], and locomotive programmable and controllable micro-system [383, 384] have been realized thanks to the materials and technologies innovations, the micro-scale geometry has limited the electronics integration. These limitations are mainly represented by temperature and chemicals sensitivity of the employed materials (such as elastomers and hydrogels) as well as their flexibility leading to substrate swelling [128, 131]. A possible way to overcome these limitations is the employment of standard rigid electronics carried on a silicon wafer that does not undergo mechanical deformation during the electronics fabrication allowing for dimensions scaling. An example of silicon electronics-based walking micro-robots with 7 nm thick Pt based electrochemical actuated legs were reported [222]. Standard electronics fabrication followed by a PDMS stamp transfer process (section 3.6) ensured a large area fabrication of floating microrobots, as shown in figures 14(a) and (b). The electronics actuation by laser powering ensured the travelling speed of the microrobot of 1 µm s⁻¹, as described in section 4.1, offering a strategy for micro- and nano- objects manipulation (figure 14(c)). Nevertheless, the micro-systems bodies consist of 5 µm thick rigid electronics, far from lying to thin-film electronics definition.

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Recently, a self-propelled flexible platform equipped with a micro-arm to grasp, transport and release micro-objects was obtained by thin-film electronics integration [241]. The micro-system was built on bilayer hydrogel/polyimide substrate to host an on-board circuit consisting of Au receiver coil and two Ti heaters in contact with Pt electrodes. Using a transfer process (section 3.6), the electronics fabrication was carried out on a rigid carrier and then released for experimental studies, where the flexibility of polyimide ensured mechanical stability to the whole system. Repeated compressive tests of the whole micro-system were performed showing the full shape recovery after each cycle. The rolled micro-engines at the edges of the system were obtained by hydrogel swelling during sacrificial layer dissolution, as described in section 4.7, forcing the bending of the polymeric substrate. Figure 14(d) depicts the fabrication steps and the final micro-system. With the same process, a micro-arm was obtained consisting of a hydrogel layer coated with a Fe/Au layer. The self-propulsion was achieved by catalytic reaction of the Pt micro-engines in a hydrogen peroxide solution. A wireless system was used to transfer the energy from an external coil to the receiver one through inductive coupling and employed for three purposes. First, it was used to heat the Pt engines through the heater allowed the locomotion control in terms of speed and directions. Next, the Fe/Au micro-arm was heated to induce rolling/relaxing (grasping/releasing) of the temperature responsive hydrogel, as shown in figure 14(g), and the transport of a Au wire was proved. Finally, the wireless energy transfer was proved by switching an integrated IR-LED showing an efficiency of 37% of the transmission/receiver energy system. The micro-robot provided a valid example of a smart and multifunctional micro-system. Although that, the use of a non-biocompatible fuel, such as H₂O₂, as well as the electronics energy demand and supply must be accounted for applications in the microscale range and, potentially, in-vivo.

A preeminent factor when fabricating electronic textiles is to identify methods of creating conductive pathways and structures in the textile. To this end, researchers have investigated the feasibility of creating simple thin-film structures such as electrodes and resistors [62, 199–201, 212, 214]. These devices were either fabricated directly on a textile substrate (an example of this is displayed in figure 16(a) [199–201, 212], created using a textile manufacturing technique [214], or transferred onto a textile structure as shown in figure 16(b) [62].

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The ability of textiles to comfortably drape over several areas of the human body enables sensors integrated on or in the textile to closely monitor important parameters of the body. Therefore, sensing textiles is one of the most prominent areas of research in electronic textiles [195]. Several textiles carrying thin-film sensors were fabricated, and in other cases, the textile itself was used as a substrate [92, 95, 96, 96, 111, 113, 146, 198, 207, 387]. Since temperature is one of the most commonly measured human vital signs, several thin-film temperature sensors were synthesised on textiles or integrated into textiles [95, 96, 111, 113, 207]. Woven polyimide temperature sensor strips with a tolerance of 2.5% were fabricated by utilising platinum as a sensing layer [95]. The bendability of polyimide substrates ensures the high flexibility of the smart textiles in a single dimension. Subsequently, Au thin films were also utilised as sensing layers on the same substrates to fabricate temperature sensors with a temperature coefficient of resistance of ${\approx}2.7 \times 10^{-3}$ ^∘C⁻¹ [96]. Similar devices to the aforementioned once were incorporated within textile yarns [111]. The temperature coefficient of resistance of the different sensing yarns varied from $2.58 \times 10^{-3}$ ^∘C⁻¹ to $1.53 \times 10^{-3}$ ^∘C⁻¹. Moreover, researchers have built temperature sensors directly on nylon yarns by depositing Au, and utilising planar and tube masks [113]. A schematic of a patterned yarn is illustrated in figure 16(c). These sensors had a temperature coefficient of resistance of $2.6 \times 10^{-3}$ ^∘C⁻¹. Textile yarns and fibers are the raw materials for creating textile fabrics, therefore, developing devices on yarns ensure smooth integration of these devices within a smart textile. A yarn-based temperature sensor with a sensitivity of $1.2 \times 10^{-2}$ ^∘C⁻¹ (range of 30 ^∘C–65 ^∘C) was fabricated by coating a PET fibre wrapped in a silk yarn with a mixture of carbon nanotubes and an ionic liquid ($[EMIM]Tf_2N$) [207]. This yarn was used to fabricate a textile glove capable of monitoring temperature.

Textile structures are able to compress, deform, and in some cases stretch to maintain their drape during bodily movements. These features are utilised to fabricate strain, tactile, and pressure sensors for wearable applications [96, 198, 207, 213, 387, 389, 390]. Woven strain gauge strips able to respond to strains from 5% to 9% were created by utilising a Au conductive layer and a polyimide substrate [96]. A different strain sensor, created by sandwiching PET fibers in between polyethylene films and then depositing conductive fillers comprising of PEDOT:PSS and silicone, demonstrated a gauge factor of 6.9 for strains up to 5% [198]. Pressure sensors able to showcase a maximum gauge factor of 10 (for compressive strain from 2% to 10%) were created by utilising a 3D printed sacrificial mold (moulding is explained in section 3.5) to surface-embed a graphene coating on a porous silicon substrate [390]. A wristband carrying the sensor was able to detect the pulse of a person having a heartbeat rate of 68 bpm. A woven capacitive pressure sensor built using two crossing silk fiber-wrapped polyurethane (PU) yarns coated with silver nanowires as electrodes and ecoflex as a dielectric insulating layer had a sensitivity of 0.136 kPa⁻¹and a relaxation time of 0.25 s [207]. A machine-washable pressure sensor, fabricated by sandwiching a PET microfiber textile layered with PEDOT in between two nickel-plated fabrics, had a response and recovery time of 52 ms and 22 ms respectively [387]. This device was able to withstand 2000 cycles of folding and the resistance of the device changed from 20 Ω to 15 kΩ when the pressure was changed from 39 kPa to 33 Pa. A capacitive/resistive touch display having a rise and fall time of 1.4 ms was fabricated by the transfer of CVD graphene onto a tape-shaped polypropylene fibre [146]. In the same work a LED display (shown in figure 16(d)) with an emission peak of 500 nm was synthesised by spin coating ZnS:Cu and BaTiO₃ onto polypropylene fibres [146]. The device was able to withstand a bending radius of 10 mm and repeated torsion ($\gt$1000 cycles). Nevertheless, these polypropylene tapes are incapable of sheering and drape which makes them less desirable for smart textile applications.

The fact that people are comfortable wearing textiles on their bodies makes it desirable to have more distinct sensors such as electronic noses within the textile structure. A textile carrying electronic nose strips were able to detect acetone (down to 50 ppm), toluene, IPA, and methanol [92]. These devices were fabricated on PEN and kapton subtrates, and they were woven into a textile. PEN substrate shows mechanical limitations similar to polyimide, where they are also unable to sheer or drape around a surface. Four different sensors were fabricated utilising non-conductive polymers (poly-iso-butylene PIB, poly-styrene PS, poly(N-vinyl-pyrrolidone) PVP and, poly-vinyl-butyral PVBU) mixed with carbon black as the conductive filler.

Smart textiles with energy sources can provide power to wearable devices [391]. Thus researchers have developed electronic textile with solar cells [119, 197] and generators that utilise piezoelectricity [208, 209], triboelectricity [47], pyroelectricity [388] and thermoelectricity [211]. Textiles carrying thin-film solar cells that convert energy of light directly into electricity were utilised for power generation applications [119, 197]. Two differently configured solar cells were fabricated using amorphous silicon and interwoven metal wires [197]. In one, glass fiber fabrics were utilised on top of the device; whereas, in the other configuration, glass fibers were utilised as the substrate. The device that was built with the glass fibres as substrate demonstrated superior performance showcasing a open-circuit voltage of 0.88 V, short-circuit current density of 3.70 mA cm⁻², fill factor of 43.1%, and an efficiency of 1.41%. Furthermore, a thin-film perovskite solar cell and supercapacitor woven into a textile had a short-circuit current density of 16.44 mA cm⁻², open-circuit voltage of 0.96 V, fill factor of 0.66, and a power conversion efficiency of 10.41% [119]. Although, the device had multiple layers comprising of PEDOT:PSS/methylammonium lead iodide(perovskite layer)/phenyl-C61-butyric acid methyl ester/Cu(OH)₂ nanotube/copper ribbon, the fabrication on a ITO/PET substrate, limits the conformability of the device around a curved surface

Textile structures are able to withstand mechanical stresses such as compression and bending. Hence, they are well suited to utilise the piezoelectric effect in order to produce electrical energy [208, 209]. Piezoelectric generators were created on woven textiles by screen printing layers constituting of a lead zirconate titanate (PZT) and silver nano particles as the piezoelectric film [209]. This film was sandwiched between two silver polymer ink electrodes. Three woven substrates polyester-cotton, cotton and polyamide-imide (Kermel) were tested. The maximum energy density under an 800 N compressive force was showcased by the Kermel textile at 34 J m⁻³, whereas cotton textile demonstrated the maximum energy density under bending at 14.3 J m⁻³. A piezoelectric yarn was fabricated utilising a three layer structure comprising of a silver-coated nylon core electrode, a circular knitted barium titanate nanoparticle and poly(vinylidene fluoride) (PVDF) nanocomposite fibers layer and a braided silver-coated nylon outer layer [208]. This yarn structure is displayed in figure 16(f). The yarn generated an open-circuit voltage of 4 V and an output power density of 87 µW cm⁻³ during cyclic compression.

A textile's ability to conform onto the human body and move with it, ensures that it can be used as a platform to harvest energy from human motion [47, 210]. A triboelectric generator capable of converting mechanical energy to electricity was synthesized by spray coating a CNT-silk layer on an electrospun silk fiber layer [210]. The device generated a power of 317.4 µW cm⁻² from hand patting (force of 20 N with a frequency of 7 Hz). Moreover, researchers have woven coaxial fiber shaped triboelectric nanogenerators to produce a current of 210 µA and a voltage of 40 V corresponding to an output power of 4 mW under a cycled compressive force of 40 N [47]. The coaxial cables were made by growing ZnO nanowires on an aluminium wire, followed by the deposition of a Au thin-film and this structure was inserted into a PDMS tube.

The close proximity of textiles to the human body enable them to harvest the heat generated by the body [211, 388]. A pyroelectric nanogenerator capable of generating a current and voltage of 2.5 µA and 42 V respectively for breath temperature changes (12 ^∘C variations) was fabricated by sandwiching a PVDF film in between two aluminium films and attached to a mask [388]. Similarly, a cotton fabric substrate coated with reduced graphene oxide and PEDOT:PSS was used to create a thermoelectric nanogenerator [211]. The device showcased a Seebeck coefficient of 25.5 µV K⁻¹ and a power factor of 0.25 µW (m K²)⁻¹ for a temperature gradient of 16.5 ^∘C.

Another crucial aspect to consider is data and power transmission for on body wearable applications. Metamaterial textiles carrying pressure sensors were able to support surface-plasmon-like modes of communication frequencies and provide a platform to propagate radio-waves around the body [389]. Metamaterials are described in paragraph 4.3. The relative signal strength indicator averaged over 32 dB for the device and it was able to transmit an output power of 20 dBm (100 mW) at a transfer efficiency of 10.5% to a loop antenna and 3.5% to a LED. This metamaterial textile improves the transmission efficiency of wireless networks by $\gt$30 dB compared to conventional radiative networks and provides secure localized measurements within 10 cm of the body.

Textiles carrying transistors provide wearable devices and electronic textiles with the requisite functionality of modern-day electronics. For smart textiles, thin-film transistors were either prefabricated and integrated onto textiles [62, 97, 204, 215, 217–219, 227], created using textile structures and enhanced fibres [142, 202, 203, 205] or synthesised directly on textile fibres [196, 206]. In most cases researchers have textile structures carrying prefabricated transistors [62, 97, 204, 215, 217–219, 227]. Early work investigated the feasibility of weaving in amorphous silicon transistors fabricated on SiNx–coated Kapton fibers [97]. These transistors demonstrated a threshold voltage of 7.5 V and a linear electron mobility of 0.13 cm² V⁻¹ s⁻¹. Moreover, a textile ribbon structure enabled the creation of transistors by gluing the gate, dielectric and semiconductor onto it [204]. Two gold wires crossing the device acted as the source and drain. The proposed transistor exhibited limited performance and only produced a drain current of around −10 nA for gate voltages of −80 V. A complex 7-Stage ring oscillator was developed on a textile as shown in figure 17(a), by using cilia-assisted transfer printing of IGZO transistors [62]. The transistor was created on a polyimide substrate utilising SiO₂ as a buffer layer and dielectric, Mo as the gate dielectric, Au/Cr layers as the source and drain electrodes and a passivation layer of SU8. The transistor demonstrated an $I_\mathrm{on/off}$ current ratio of 10⁸. In the case of the ring oscillator at a V$_{DD}$ of 10 V the output voltage oscillated with a frequency of 33 kHz and a propagation delay of 2.2 µs.

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The cylindrical surfaces of metal wires and optical fibers also enabled the fabrication of transistors [215, 217–219, 227]. Maccioni et al created an organic field-effect transistor by evaporating pentacene on a metal wire and created two devices one with source drain contacts made of gold and the other with PEDOT:PSS. Both the devices had an $I_\mathrm{on/off}$ current ratio of 10³ and the transistor created with PEDOT:PSS showcased the best performance demonstrating a 0.06 cm² V⁻¹ s⁻¹ while functioning at a threshold voltage of 0.6 V. Thin metal wires are well suited for smart textile applications because of their ability to undergo multi-axial bending. Indeed, heated copper wires with a copper oxide shell structure with the source drain electrodes evaporated on it as displayed in figure 17(b), was utilised to form a transistor [215]. A woven structure was used to connect perpendicular wires to the source and drain electrodes. The device had an $I_\mathrm{on/off}$ current ratio of ${\gt}10^{4}$ and a channel mobility of 26.3 cm² V⁻¹ s⁻¹. However, it had a gate leakage current which made it undesirable for developing circuits. Nonetheless, the device demonstrated an 82% change in drain current when the relative humidity was varied from 0% to 70% making it a potential humidity sensor. Subsequently, transistors were also fabricated on optical fibers [217–219], although their limited flexibility makes them less desirable for most smart textile applications. These devices were fabricated by depositing chromium and AlO_x on the optical fiber for the gate electrode and dielectric respectively. Park et al dip coated it with a InO_x or IGZO precursor solution [217]. The source/drain electrodes were made by depositing aluminium and the device showcased a field-effect mobility and an $I_\mathrm{on/off}$ current ratio of 3.7 cm² V⁻¹ s⁻¹ and ${\gt}10^{6}$ respectively. In contrast, Heo et al utilised SWCNT network as the channel layer [218, 219]. Au electrodes were utilised with non-isolated and isolated SWCNT channels to achieve an $I_\mathrm{on/off}$ current ratio of 10⁵ and a field-effect mobility of 3.61 cm² V⁻¹ s⁻¹ [219]. Alternatively, in another paper dry spun conductive carbon nanotube fibers were sewn in as source/drain electrodes [218]. This device had a saturation mobility of only 0.56 cm² V⁻¹ s⁻¹. Nonetheless, a pressure sensor was developed using this transistor which had a sensitivity of $3.86 \times 10^{-1}$ Pa⁻¹ and $2.4 \times 10^{2}$ Pa⁻¹ for pressure ranges of 0–32 Pa and 32–280 Pa respectively.

Researchers have combined multiple fibres with various textile structures to construct transistors for smart textiles [142, 202, 203, 205]. Organic transistors were fabricated by adding a drop of polymer electrolyte at the junction of two PEDOT/PSS coated fibres [202]. This structure is illustrated in figure 17(c). The coated fibres were running perpendicular to one another and a woven textile was utilised to realise the structure. The transistors had an $I_\mathrm{on/off}$ current ratio ${\gt}10^{3}$ for gate voltages ranging from 0 and 1.5 V. These transistors were utilised to create an inverter and multiplexer. A similar transistor was fabricated by twisting two PEDOT:PSS coated kevlar yarns and putting a drop of electrolyte [203]. This new method of creating transistors utilising twisting ensured that the device can be integrated more easily into different textile structures rather than just woven once. In this way, an analogue amplifier with an amplification of 7.5 was created and the transistor showcased an $I_\mathrm{on/off}$ current ratio of ${\gt}10^{3}$. A more recent publication has fabricated a transistor utilising three twisted Au microfibers [142]. Here, for the source and drain electrodes the microfibers were coated in P3HT solution, then twisted together and an ionic gel was dropped on the twisted structure. Thereafter the Au microfiber for the gate electrode was wound around ion-gel and then passivated using Ecoflex. The device showcased $I_\mathrm{on/off}$ current ratio of $1.46 \times 10^{5}$ at drain and gate voltages below −1.3 V. The device had a transconductance and a threshold voltage of 111.8 µs mm⁻¹ and −1.01 V respectively, these values were maintained up to 80% after bending over a radius of 2.0 mm. Biosensors were achieved by creating a transistor comprising of a source drain synthesised of different ion selective membranes around a PEDOT:PSS soaked acrylic textile yarn substrate and a Ag/AgCl wire for the gate electrode [205]. One device had a potassium selective membrane solution and was able to detect concentrations of potassium from 10⁻⁴ mol to 1 mol with a selectivity of 20.34%. The other transistor had a calcium-selective membrane and was able to detect calcium ions in concentrations ranging from 10⁻⁴ mol to 10⁻¹ mol with a selectivity of 12.19%. Both the transistors also responded to changes in sodium concentration.

Textile fibers can be directly employed as substrates when fabricating transistors [196, 206]. Devices engineered on nylon and glass fibers showcased limited performance with an $I_\mathrm{on/off}$ current ratio of ${\approx} 3 \times 10^{2}$ and 10⁴ (functioning only in depletion mode with a threshold voltage of −12.5 V) respectively [196]. In the same work devices fabricated on a silicon wafer were transferred to textile fibers utilising a parylene membrane and these transistors demonstrated an $I_\mathrm{on/off}$ current ratio of 10⁷ and a field-effect mobility of 7.2 cm² V⁻¹ s⁻¹. A microscopic image of a transistor synthesised on a nylon yarn is displayed in figure 17(d). In a different work, a p-type transistor was synthesised by drop casting a semiconductor solution on a linen thread and then knotting Au wires as the source and drain contacts [206]. Two semiconductor solutions were tested CNTs and organic semiconductor P3HT (regioregular poly(3-hexylthiophene)). They both had limited $I_\mathrm{on/off}$ ratios of ${\gt}10^{2}$ and the transistor made with CNT had a linear mobility of 3.6 cm² V⁻¹ s⁻¹.