# Perspective on GRM based materials for e-textiles and flexible large-area electronics

Maria Smolander, Andreas Roepert, Awa Garlinska, Laura S van Hazendonk, Heiner Friedrich, Tim Biemelt, Antonio Gaetano Ricciardulli, Khaled Parvez, Marco Pelin, Paolo Samorì, et al.

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## To cite this version:

Maria Smolander, Andreas Roepert, Awa Garlinska, Laura S van Hazendonk, Heiner Friedrich, et al. Perspective on GRM based materials for e-textiles and flexible large-area electronics. 2D Materials, 2025, 12 (2), pp.023001. 10.1088/2053-1583/adabf3. hal-04923318

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Submitted on 31 Jan 2025

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PERSPECTIVE • OPEN ACCESS

# Perspective on GRM based materials for e-textiles and flexible large-area electronics

To cite this article: Maria Smolander et al 2025 *2D Mater.* **12** 023001

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PERSPECTIVE

# Perspective on GRM based materials for e-textiles and flexible large-area electronics

Maria Smolander<fnref n="1" />,*, Andreas Roepert<fnref n="2" />, Awa Garlinska<fnref n="2" />, Laura S van Hazendonk<fnref n="3" />, Heiner Friedrich<fnref n="3" />,<fnref n="4" />, Tim Biemelt<fnref n="3" />, Antonio Gaetano Ricciardulli<fnref n="5" />, Khaled Parvez<fnref n="7" />, Marco Pelin<fnref n="8" />, Paolo Samori<fnref n="6" />, Cinzia Casiraghi<fnref n="7" />, Henrik Sandberg<fnref n="1" />, Teemu Ruotsalainen<fnref n="1" /> and Neil Chilton<fnref n="9" />

E-mail: maria.smolander@vtt.fi

**Keywords:** GRM, e-textiles, printed electronics, flexible electronics, graphene

## 1. Introduction

One of the targets of the Graphene Flagship project (graphene-flagship.eu) was to demonstrate through functional prototypes the viability and performance of graphene in large-area applications where flexibility, stretchability and sustainability play a key role. As an alternative to metal-based conductors, e-textile fibres based on or modified with printed graphene or graphene-related materials (GRM) were investigated together with graphene-based functionalities for e-textile applications where mechanical properties have a huge importance. During the project several GRM-based building blocks were realized for applications in sports, well-being and health monitoring. This part of Graphene Flagship focussing on flexible electronics relied on the contribution by several research partners including VTT Technical Research Centre of Finland (FI), CEA—Commissariat à l'énergie atomique et aux énergies alternatives (FR), CRN—Consiglio Nazionale delle Ricerche (IT), ICFO - The Institute of Photonic Sciences (ES), University of Wien (AT), Eindhoven University of Technology (NL), University of Cambridge (UK), University of Lille (FR), The University of Manchester (UK), University of Strasbourg (FR), Aalto University (FI), and industrial partners FlexEnable (UK), Graphenea (ES), Nokia (UK), Novalia (UK), Printed Electronics Limited (UK), Interactive Wear (DE), Indorama Ventures (DE) and Qurv (ES).

We carried out a market study on the e-textile applications of graphene to evaluate the feasibility for implementing GRM-based materials for e-textiles. The key objective of the market study was to understand the e-textile market characteristics and dynamics, its major application areas, and the requirements of technology and materials for that market, as well as to investigate the market potential of graphene-containing fabrics and graphene-based wearables.

Conductive, flexible and lightweight fabrics have application possibilities e.g. in thermal management, electrostatic protection, or biomonitoring. For biomonitoring in health and fitness, measurement of pressure, strain, or bio-signals in wearable sensors, smart garments, or health monitoring devices are of particular interest. Graphene-based materials have the potential to make significant improvements in e-textile applications and large area electronics because of the outstanding electrical and thermal conductivity, strength, and flexibility of graphene. Moreover, graphene's biocompatibility allows it to be more suitable for close-to-the-body applications than metal-based e-textiles. Graphene also has potential to improve environmental impact of e-textiles, especially for non-recycled applications such as those in medical areas. We propose heating and sensor applications as most promising implementation possibilities, both of which belong to the most relevant e-textile market segments.

<footnote n="1">¹ VTT Technical Research Centre of Finland, PO Box 1000, 02044 VTT, Espoo, Finland</footnote>

<footnote n="2">² Interactive Wear AG, Petersbrunner Str. 3, D-82319 Starnberg, Germany</footnote>

<footnote n="3">³ Laboratory of Physical Chemistry, Department of Chemical Engineering and Chemistry & Institute for Complex Molecular Systems (ICMS), Eindhoven University of Technology, PO box 513, Eindhoven, MB 5600, The Netherlands</footnote>

<footnote n="4">⁴ Center for Multiscale Electron Microscopy, Eindhoven, The Netherlands</footnote>

<footnote n="5">⁵ Indorama Ventures Fibers Germany GmbH, Forster Straße 54, 03172 Guben, Germany</footnote>

<footnote n="6">⁶ Université de Strasbourg, CNRS, ISIS, 8 allée Gaspard Monge, Strasbourg 67000, France</footnote>

<footnote n="7">⁷ Department of Chemistry, University of Manchester, M13 9PL Manchester, United Kingdom</footnote>

<footnote n="8">⁸ Department of Life Sciences, University of Trieste, 34137 Trieste, Italy</footnote>

<footnote n="9">⁹ Printed Electronics Ltd, Amphenol Invotec Building, Hedging Lane, Tamworth, Staffordshire B775HH, United Kingdom</footnote>

<footnote>* Author to whom any correspondence should be addressed.</footnote>
Today, graphene-based textiles are a niche market. Also, in graphene containing applications, the electronic functionality is a small niche, with most products focussing on heat management, anti-bacterial surfaces, and mechanical properties. The prerequisite for sustainable growth of GRM-based e-textiles is the development of stable, highly conductive material for low-resistance textile heating, electrode material, or sensors, which could optimally be produced in a roll-to-roll process or using regular textile processing equipment. The requirement for low electrical resistance in wearables is driven by the fact that these applications are typically battery-powered, so for heating applications the voltage will be low and therefore the resistance must also be low. A breakthrough for the mass market can also only be achieved with the development of effective and efficient postprocessing technology, including cutting and contacting/joining, providing components ready for integration. Encapsulation or shielding methods are also prerequisites for the successful implementation of electronic or functional materials in textiles.

For other biomonitoring fields like strain- and pressure sensors, low resistivity is not as important as the actual performance of the sensors, their stability of sensing properties over time, and in various environments (chemical, temperature, humidity). In summary, the application potential highly depends on the achieved performance of the materials and their integration technologies, as well as their capability to maintain natural textile properties like flexibility, breathability, and washability, and adding other features such as mechanical strength or abrasion resistance.

In the following sections we discuss the Graphene Flagship work on GRM-containing conductive textile fibres, yarns, and fabrics for e-textiles as well as GRM-based functionalities for e-textile applications. Particular focus is put on the fabrication of GRM-containing textiles through two different methods, namely (1) printing and (2) direct incorporation of GRM into textile fibres. Graphene-based functionalities for wearables like heating elements, strain and respiration sensors, electrodes for biopotential measurement, and demonstrators with these functionalities are described followed by examples of characterization of the developments in terms of washability and skin compatibility. Finally, development efforts towards electronic components will be briefly tackled.

## 1.1. GRM-based printed flexible conductors

Printing-based additive manufacturing methods enable material and energy efficient production of electronics. Especially for large-area applications like wearables or RF passives [1] where printing of conductors provides more sustainable, flexible,

stretchable and cost-effective alternatives to traditional techniques. In this work, a high-performance graphene nanoplatelet (GNP)-based ink formulation with a skin-compatible, thermoplastic polyurethane (TPU) binder was developed primarily for screen printing (figure 1(a)) [2]. Sheet resistances down to 34 Ω/(□·mil) ("mil" herein meaning 25 μm printed thickness) could be obtained as printed, which could be considered highly competitive with other GNP-based inks containing GNPs produced through liquid-phase exfoliation (LPE) [3–7]. Moreover, the conductors remained conductive even under 100% strain (figure 1(b)). Importantly, the GNP-based alternative remained highly stable over 1000 stretch cycles of 20%–50% strain (figure 1(c)), corresponding to the strain levels experienced by the human skin. This durability provides a great advantage over printed silver-based conductors containing silver flakes or silver nanowires as conductive fillers, which generally suffer from their resistance steadily increasing and/or even exploding after a few hundred cycles of 10%–50% strain [8–11]. Although our baseline sheet resistances are higher, they offer superior cyclic stability. Furthermore, photonic annealing with intense pulsed light was successfully used to further reduce the sheet resistances of the conductors on thermally sensitive substrates to remarkably low levels below 10 Ω/(□·mil). Unlike conventional post-processing techniques such as thermal annealing, photonic annealing does not damage the stretchable substrate thanks to the high temperatures required for annealing only being reached for very short time scales on the order of milliseconds [12, 13]. As a consequence, the stretchability and cyclic stability of the conductors was preserved in our study (figure 1(d)).

Thanks to the easy tunability of the ink rheology, high conductivity, stretchability and stability of the printed tracks, the material has wide application potential in wearable technology based on different printing-based deposition methods. Direct ink writing (DIW) was used as example to demonstrate the potential of these GNP-inks for high-resolution 3D printing [14]. Generally, inks containing nonspherical conductive fillers like GNPs present a challenge for high-resolution printing due to the risk of nozzle or print screen clogging. To this end the printing process and ink formulation were simultaneously optimized via Design of Experiments to derive a predictive understanding of filament quality and morphology. By tuning the nozzle-to-substrate distance, print speed and ink flow, filaments with a wide range of dimensions (height and width) could be printed even though the nozzle size was fixed (figure 1(e)). This resulted in high-quality prints with sub-nozzle diameter print widths of <100 μm and conductivities suitable for application in sensors or electrodes. As an application example for DIW printed graphene
<figure><img src="image_5.png" /><figcaption>**Figure 1.** Stretchable graphene nanoplatelet (GNP)-based ink and its electromechanical behaviour, printability and applications. (a) Schematic of the ink production process starting from thermally expanded graphite through high-shear exfoliation in ethyl acetate (EtAc)/isopropyl alcohol (IPA) in the presence of ethyl cellulose (EC), followed by the addition of a stretchable TPU binder and solvent exchange into propylene glycol ether (dowanol); (b)–(d) normalized resistance (R/R₀) of 76 × 1 mm straight samples screen printed on TPU substrates (b) exposed to strains with linearly increasing strain amplitude from 2% to 100% in 50 steps; (c) during cycles 983–1000 of a 1000-cycles-long cyclic strain test with strain levels cycling between 0 and 20%; (d) normalized resistance at the maximum strain level of each cycle during a 1000-cycles-long cyclic strain test with strain levels cycling between 0 and 20% for samples post-treated with varying levels of photonic annealing to gradually reduce the initial resistance R₀ from 34 to 8.9 Ω/(□·mil); (e) tunability of the filament width/nozzle diameter ratio in DIW by adjusting the print parameters as investigated via Design of Experiments; the bar chart shows the distribution of all filament width-to-nozzle-ratios obtained across the study; (f) robotic gripper device consisting of four individually-controllable ‘fingers’ from liquid crystal network actuators with printed graphene-based resistive heating elements. In the picture, the device is already holding onto a small object (polystyrene block of 70–100 mg), and in the process of gripping a second object (strip of paper). (a)–(d) Reprinted with permission from [2]. Copyright (2022) American Chemical Society. (e) Reprinted from [14], Copyright (2024), with permission from Elsevier. (f) Reprinted with permission from [15]. Copyright (2024) American Chemical Society.</figcaption></figure>

conductors, two soft robotic devices were designed and realized. These devices composed of four individually controllable liquid-crystal network actuators, which are thermally actuated by the graphene-based heating filaments [15], were shown to carry out complex gripping actions similar to a human hand (figure 1(f)). These all-organic soft robots which are stable in humid and saline environments, have potential applications in surgical devices, biomedical applications and wearables.

## 1.2. GRM-based conductive yarn

In the project, a GRM-based conductive core-shell type of PET-yarn was successfully developed all the way from base GRM production, through compounding of carbon black and graphite nanoplates and melt-spinning in a relatively large pilot-scale facility with corresponding annual production of 5–15 tonnes of conductive yarn with good performance and durability. This conductive yarn has a monofilament-like appearance and is currently
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Stress–strain hysteresis behaviour of GRM based PET-yarn manufactured at Indorama Ventures.</figcaption></figure>

producible in a range of 0.3–0.7 mm diameter, with corresponding resistances of 5000–1000 Ω cm⁻¹. This type of electrically conductive yarn provides a unique set of characteristics. It shows extraordinarily high wear tenacity up to 70 cN/tex (where tex is the mass of 1000 m of the fibre expressed in grams), while maintaining 12%–14% elongation at break and 0% shrinkage at 120 °C. Due to the nature of shell and core material, respective threads show elastic behaviour up to 10% elongation for more than 100 cycles in stress–strain-hysteresis investigation (see figure 2). The material can be bent more than 15 000 times without any damage to the surface or electrical characteristics. Concerning resistance against mechanical abrasion, electrical conductivity is slightly decreased during bending-abrasion testing. The small change in conductivity (figure 2) can be ascribed to a minimally increased surface roughness after 150 strokes at 150 cN, showing the high mechanical resistance of the material. With respect to potential outdoor applications, the yarn was also characterized in an artificial exposure chamber. A standard semi dull PET-yarn was used as a reference. Results are displayed in figure 3. While standard PET-yarn shows decrease of tenacity and elongation at break upon artificial weathering, the electrically conductive yarns are stable against exposure to UV-radiation and water.

The GRM-based threads were woven or knitted into different cuttable and interdigitated conductive fabrics (figure 4), which were then further implemented for demonstrators. They were characterised in washing tests complying with AATCC-135-2010 standard as described later in this paper.

## 1.3. GRM-based heating applications for e-textiles

Today heating applications dominate the e-textile market, the main segment being transportation,

e.g. automotive seat heating. In this segment the main challenge is cost-efficiency. In particular, integration costs need to be minimized. Hence, we were also looking for specific segments with opportunities for GRM. It was concluded that these opportunities for GRM-based textile materials could be the integration of heating components with sensing, especially occupancy and temperature. For wearable heating, a flexible, elastic, and robust heating component for e.g. gloves is a good example, providing benefits compared with existing components of embroidered, woven, or knitted material. In the area of therapeutic heating, electric heating elements offer the possibility to control the temperature as a major benefit compared to chemical or pre-heated elements. Graphene could provide better performance (time to heat-up) and integration with temperature sensors. We realized different wearable heating demonstrations based on project developed and commercial graphene materials (figure 5).

## 1.4. GRM-based sensing applications for e-textiles

As reviewed by Chen *et al* [16] a variety of graphene-based flexible or stretchable sensors have been developed by incorporating graphene or graphene derivatives into the stretchable substrate for strain sensing. As Chen *et al* summarize, the good mechanical properties and strong responses to external strains of GRMs enable their usage in high-sensitivity electro-mechanical sensors. Compared to metal-based components, graphene has the potential to add benefit to these solutions due to the absence of corrosion when exposed to sweat, and the expected resistance to mechanical stress from washing. We investigated the use of printed GRM meanders for respiration sensing. In this demonstrator GRM meanders were printed on top of thin TPU, which
<figure><img src="image_7.png" /><figcaption>**Figure 3.** Mechanical stability of GRM-based conductive, core-shell type of PET-yarn (350 and 650 µm) in comparison to standard PET-yarn under artificial exposure according to DIN EN ISO 105—B10 (60 W m⁻² after 750 h), results displayed in per cent of initial level.</figcaption></figure>

a)

<figure><img src="image_8.png" /></figure>

b)

<figure><img src="image_9.png" /><figcaption>**Figure 4.** (a) Woven fabrics contacted via ultrasonic welding (350 µm thread, resistivity 2.2 kΩ, 10 × 10 cm²) (b) pressure sensors based on GRM fabric.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 5.** Heating demonstrator integrated by Interactive Wear (graphene material from NANESA).</figcaption></figure>

allowed the needed low force stretching and full recovery to their original length. After the printing step, GRM meanders were encapsulated with another layer of TPU and finally heat laminated to a t-shirt as shown in figure 6. Breathing while wearing

the t-shirt changed the conductivity of the GRM meanders, which was monitored and visualised via a mobile phone app. We also investigated the humidity-tolerance of the graphene electrodes integrated in this t-shirt, which is relevant in case of exposure to sweat
<figure><img src="image_11.png" /><figcaption>**Figure 6.** Integration of GRM-based electrodes and two respiration sensors into a shirt (integration by Interactive Wear).</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 7.** Humidity-tolerance of printed graphene electrodes like the ones in figures 1 and 6. The figure shows the resistance normalized by the initial value ($R/R_0$) over a humidity cycle in which the humidity was increased from 30%–90% and subsequently lowered back to 30% for two independent samples. The final resistance was only slightly higher than the initial value.</figcaption></figure>

or washing. As is shown in figure 7, the electrodes are highly robust to changes in humidity. Even after exposure to 90% humidity the resistance increased only 12 ± 7%, being small in comparison to the magnitude of the strain response ($R/R_0$ > 400% at 20% peak strain, see figure 7). Therefore, exposure to humidity should not interfere with respiration or strain sensing applications.

A highly sensitive pressure sensor developed by the University of Strasbourg could potentially replace the typical optical PPG sensor, providing lower costs and power consumption for health monitoring and digital healthcare. Here, reduced graphene oxide (rGO) was embedded in a trifunctional poly-benzoxazine thermoset resin matrix (PtPA) to yield graphene-based piezoresistive pressure sensors with tuneable sensitivity [17]. The synthesized electrically conductive blend enabled the modulation of piezoresistive response to pressure. As a result, the fabricated pressure sensors exhibited high sensitivity between $10^{-2}$ to 1 kPa$^{-1}$, which can be modulated by controlling the rGO:PtPA ratio, as well as the curing temperature.

We also developed chemical vapour deposition (CVD) graphene based dry electrodes with direct skin contact, eliminating the need for wet-gel application. These electrodes for the biometric ECG measurements comprise a bilayer with copper or silver foil on top of which a CVD graphene monolayer is either grown or wet transferred. A Finnish patent was granted for the electrodes and the corresponding manufacturing process [18]. Dry ECG electrodes were integrated into e-textiles and successfully benchmarked against commercial electrodes. The electrodes were observed to have relatively low impedance around 50%–70% of the respective values of commercial electrodes (3 M 2239), and good signal
<figure><img src="image_13.png" /><figcaption>**Figure 8.** Contact impedance of Cu/Ag/graphene electrodes compared to state-of-art gel electrodes (3 M 2239).</figcaption></figure>

quality with superior signal to noise ratio as shown in figures 8 and 11. These observations suggest that CVD graphene has a high potential for use as a dry electrode material, if the suitable integration techniques are used. For application in a t-shirt, we used silver-based conductive glue to join textile cables onto copper foil plate with silver and CVD graphene, which yielded very good results.

As a final demonstrator that brought together many of the elements described above, an ECG monitoring t-shirt was designed and assembled by the partners (figures 9 and 10). This shirt utilised dry graphene electrodes described above, together with a fully printed wiring harness that was printed and laminated on the inside surface of the garment. This approach demonstrated the possibility to realize a wearable device for ECG and EMG applications using washable, resilient, and stretchable printed wiring based on a transfer print method. The signal quality demonstrated by the t-shirt was very good, while being comparable to high-quality commercial gel electrode references (Kendal H135SG) as shown in figure 11. GRM materials were implemented as the dry electrodes described above, or as part of the flexible wiring itself. The electronic module used in the demonstrator included 5-wire (4 lead) ECG and 2 channel EMG and communicated via Bluetooth.

## 1.5. Washability of GRM-based e-textiles

Despite tremendous progress in wearable electronic textiles, stability and durability concerns strongly

hinder their applicability into the consumer market [19]. In this regard, washability is one of the most crucial requirements for wearable applications. Washing requirements from apparel industry vary from 20 cycles handwash for outer jackets, to 50–150 cycles machine wash at 60 °C for sports and underwear. The most challenging requirements are from workwear, especially from workwear leasing companies using industrial laundry up to 90 °C.

In the Graphene Flagship we benchmarked the electrical performance (i.e resistance *R*) of graphene-based fabrics fabricated by our research partners before and after washing, using a top-load washing-machine (SDLATLAS Vortex M6) according to the AATCC-135-2010 standard. Conductive yarns described in the previous chapter with diameters of 340 and 650 µm were tested with different washing cycles (delicate, normal, permanent press) and temperatures (27, 41, 49, 60 ± 3 °C). Washing tests were performed using 1993 AATCC standard reference detergent and Type 1 laundering ballast. Washing tests on both type of yarns using delicate cycles at 27° ± 3 °C, revealed that the conductive core-shell yarn did not degrade after 10 washing cycles and retained its electrical behaviour, thus being compatible with consumer market and industrial needs. However, for several applications, especially for the most challenging conditions, reliable material-specific connectivity must be developed and tested along with the material washability.
<figure><img src="image_14.png" /><figcaption>**Figure 9.** T-shirt prototype presented at the final review of Graphene Flagship in Gothenburg 2023. The inner conductive traces consist of stretchable silver ink that is protected by a further layer of printed graphene ink that was found to improve washing resilience.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Figure 10.** T-shirt testing with ECG data transmitted to the PC application. A more complete explanation of the t shirt and concepts can be seen on YouTube (https://youtu.be/bOCnQ0sSFQc).</figcaption></figure>

In addition to the GRM-based yarn and fabric, the washability of several other materials was also investigated based on the standardized method described in AATCC-135-2010. For instance, reduced graphene oxide (rGO)-coated electrospun polyamide (PA6) and cotton mats did not exhibit degradation of resistance (R), thus being compatible with consumer market and industrial needs. In contrast, other graphene-based impregnated or coated fabrics exhibited R increase in the range of 3%–18% after washing.

**1.6. Skin compatibility of GRM-based materials**

A thorough safety assessment is a prerequisite for the use of graphene or any other nanomaterials in

consumer products. The first step in such assessments are *in vitro* studies, using a panel of relevant cell lines and investigating the internalization profile and cytotoxicity. This assessment has been done for the first time on water-based 2D inks, including graphene, MoS₂, WS₂, and hBN, enabled by supra-molecular chemistry [20] and it is becoming a common approach for the development of new 2D material inks. However, further research such as studies on elimination and biodegradation pathways of the nanosheets is needed to enable development of safe 2D material based inks [21, 22]. Very recently, a study on the graphene oxide safety profile and on human exposure has shown that highly purified graphene
<figure><img src="image_16.png" /><figcaption>**Figure 11.** ECG signals recorded using commercial wet gel electrodes H135SG via copper leads (blue) and the ECG shirt demonstrator with graphene coated dry electrodes and screen printed silver traces on stretchable fabric (orange).</figcaption></figure>

oxide nanosheets do not produce acute changes in respiratory or cardiovascular function, nor systemic inflammation [23], hence advancing our understanding on the safe use of 2D materials.

When dealing with e-textiles and/or devices in direct contact with the skin, safety assessment after cutaneous exposure is crucial [24]. For GRMs, good knowledge has been gained regarding their safety at the skin level using both *in vitro* and *in vivo* approaches, to a large extent through activities carried out in the frame of Graphene Flagship [22].

The majority of the *in vitro* studies were carried out on keratinocytes or fibroblasts. Studies on HaCaT skin keratinocytes revealed significant and only partially reversible [25] cytotoxic effects only after long exposure times (48–72 h) at high concentrations of GRMs (*>10 mg ml⁻¹*), showing that the highest oxidized materials such as GO were the most cytotoxic [26]. The mechanism of toxicity seems to involve mitochondrial membrane depolarization and production of reactive oxygen species [27–29] with consequent rearrangement of cellular metabolome [30, 31] and a pro-inflammatory response exerted at sub-cytotoxic concentrations [32]. *In vitro* studies were carried out on more predictive models such as 3D reconstructed human epidermis, following robust and reliable test guidelines (TG) given by the Organization for Economic Cooperation and Development (OECD), excluding both irritation [32, 33] and corrosion [34] properties for a wide range of different GRMs including GO, rGO, FLG (few-layer graphene), GNP and others. Similar conclusions were obtained from tests with 3D reconstructed human

epidermis on the above-mentioned CVD based dry electrodes for ECG measurement. The electrodes constituted Cu thin film (20 and 75 µm) topped with a single layer of CVD graphene for low contact impedance of electrodes. In this work with the dry electrodes, the adoption of the OECD TG 439 excluded skin irritation properties (unpublished results). Histological analysis revealed the presence of small GRM depots within the outer layers of the epidermis, suggesting the possibility for GRMs to penetrate into the skin [32], as subsequently confirmed for GO by using Franz diffusion cells [35].

Regarding the *in vivo* studies, only limited data are currently available. Two independent studies investigated the skin sensitization potential of GNP [36] as well as FLG and GO [37] in female mice after topic exposure following procedures described in the OECD TG 442B. Both studies demonstrated the tested materials as non-sensitizers, FLG and GO in particular, without signs of skin irritation and inflammation, despite their capability to slightly penetrate the skin [37]. On the whole, these results highlight a good biocompatibility at the cutaneous level for different GRMs, supporting the safe use for GRM-based technological applications directly in contact with the skin.

## 1.7. Future possibilities in electronic components, devices and circuits

The Graphene Flagship also contributed to the development of printed electronic components such as transistors and diodes, as well as towards flexible devices and circuits based on graphene and 2D
materials. Wearable electronics and the Internet of Things require new electronic materials that can achieve performance suitable for these applications whilst enabling reductions of the fabrication costs and power consumption, as well as simple device integration onto flexible substrates. 2D materials are very attractive candidates for active components needed for these applications, such as transistors and diodes [38, 39].

Within the Graphene Flagship project, active and flexible electronic devices were fabricated by using biocompatible and water-based inkjet printable 2D material-based inks [20]. Graphene, hexagonal Boron Nitride (h-BN) and transition metal dichalcogenides (TMDs) were exfoliated in water by LPE, assisted using pyrene derivatives [40]. Raman spectroscopy and transmission electron microscopy revealed that the produced nanosheets have high crystallinity. Statistical analysis by atomic force microscopy shows that the average lateral size is 221 nm for graphene and ~50 nm for TMDs, while the average thickness of the graphene and TMDs nanosheets are 6.8 and 4.4 nm, respectively [41]. Furthermore, quantitative structural information on the exfoliated 2D inks were obtained by x-ray photoelectron spectroscopy and synchrotron x-ray based characterisation techniques [42]. Thermal properties of the exfoliated 2D inks measured through frequency domain thermoreflectance method revealed high in-plane and remarkably low out-of-plane thermal conductivity values, giving rise to a high anisotropy [43]. These inkjet printable inks have been used for the fabrication of various types of printed devices such as photodetectors (PDs) [20, 44, 45], memories [20, 46], diodes [47–49], capacitors [50] and transistors [48, 51–53] on both rigid and flexible substrates, fully printed electrochemical sensors where graphene is used to replace the printed silver gate electrode [54], and wearable humidity sensors based on supramolecular functionalization of h-BN nanosheets [55].

A fully printed PD was made on paper by printing $MoS_2$ between printed bottom and top graphene electrodes, giving rise to the heterostructure: $GrB/MoS_2/GrT$ (figure 1(a)). The PD shows a photoresponsivity of above 1.5 mA W⁻¹ at 514.5 nm under a laser power of 1.60 mW, which is stable under bending conditions up to 2% strain (figures 12(b) and (c)). The photoresponsivity of the PD can be increased by several orders of magnitude by replacing the $MoS_2$ ink produced by LPE with semiconducting 2D materials (e.g. $MoS_2$ and $HfS_2$) produced by electrochemical exfoliation (ECE) [56–58].

Furthermore, printed metal-insulator-semiconductor (MIS) diodes were fabricated on glass substrates using semiconducting carbon nanotubes (CNTs) and anatase $TiO_2$ nanosheets produced by bottom-up approach [59], printed between two silver contacts (inset of figure 12(d)) [48]. As shown

in figures 12(d) and (a) device fabricated using a printed $TiO_2$ film ~360 nm thick, displays the typical I–V characteristics of a diode, exhibiting rectifying behaviour with a rectification ratio (r) of approximately 100 at 20 V. By reducing the $TiO_2$ layer thickness to ~170 nm, the diode still shows a forward current density and strong rectification (r ≈ 130) even at a low voltage of 5 V (figure 12(e)), making it well-suited for low voltage applications. [48]

Inkjet printing is a flexible technique that can be combined with other techniques such as CVD. For instance, single-layer crystals produced by CVD can be transferred and integrated into fully printed devices to act as an ion barrier and to reduce hysteresis [46]. In addition, water-based 2D material inks can be printed onto 2D crystals produced by CVD without affecting the electronic properties of the CVD crystal. For example, hybrid thin film transistors utilizing CVD grown $MoS_2$ as the channel material and printed hBN and Ag as dielectric and electrodes, respectively, in a top gate configuration (figure 12(g)), exhibit an average field-effect mobility of 5.5 cm² Vs⁻¹ with minimum hysteresis and an average on-off ratio of 8 × 10³ (figure 12(h)). The device is stable under bending up to a bending radius of 8 mm (figure 12(i)). This fabrication method has been successfully adopted to create complex integrated circuits, such as high-gain inverters, logic gates, and current mirrors, all made on paper, by exploiting the ability to print and connect several transistors with the inkjet printer [51]. This technology can also be seamlessly integrated with fully printed p-type CNT-based transistors, enabling the successful fabrication of CMOS inverters with a gain of up to 8, a noise margin of approximately 50% and a static power consumption of ~30 nW (figures 12(j)–(l)). [53] Finally, the approach can be extended to other 2D material-based inks. For example, $TiO_2$ anatase nanosheets were used in place of the h-BN based inks to print the gate dielectric onto CVD $MoS_2$, used as the channel of a hybrid field-effect transistor fabricated on paper (inset of figure 12(f)). [48] As shown in figure 12(f), the on-off ratio of the transistor is more than three orders of magnitude at low applied voltages (<3 V), and the leakage current ($I_G$) remains below 2 nA across the entire voltage range because of the excellent dielectric properties of the printed $TiO_2$ films.

Additionally, 1/f noise measurements were performed on hybrid FETs made with bilayer CVD $MoS_2$ transferred onto paper followed by inkjet printed hBN gate dielectric and Ag contacts [52]. A detailed 1/f noise analysis revealed that the noise characteristics of bilayer $MoS_2$ FETs are not compromised by the use of paper as substrate and by the printing of the Ag contacts. Tests conducted under bending stress show that the devices remain stable, with noise level unaffected by either tensile or compressive strain down to a curvature radius of 12 mm.
<figure><img src="image_17.png" /></figure>

## 2. Conclusions

The Graphene Flagship aimed to demonstrate the viability and performance of graphene in e-textile applications where mechanical properties such as flexibility and stretchability as well as sustainability

have a crucial role. We clearly recognize that for graphene-based solution deposited materials it is practically not possible to reach the figures of merit in conductivity provided by metallic nanoparticles [38]. This challenge was tackled through market study-based application case selection and design,
and the realization of functional prototypes relying
on the manufacturing methods of printing and dir-
ect incorporation of GRM into textile fibres. The mar-
ket study concluded that the most promising oppor-
tunities for graphene in e-textile applications would
be found in heating and sensor applications, and
hence demonstrators in these applications were pur-
sued. To incorporate graphene into e-textiles we suc-
cessfully developed a GNP containing tuneable and
stretchable ink suitable for printing on elastomers and
even for DIW. Furthermore, incorporation of GRM
into fabrics was successfully carried out through a
compounding process resulting in conductive yarn
which is mechanically stable and could be utilised
with standard textile industry equipment. This yarn,
among other investigated materials was found to
retain its electrical behaviour in standardized washing
tests, thus being compatible with the consumer mar-
ket and industrial needs. In addition, we successfully
investigated the application of GRM based material
in various functional concepts like e-textiles for heat-
ing, graphene-based pressure sensors and respiration
sensors, and dry electrodes for ECG monitoring. A
wearable demonstrator incorporating skin compat-
ible dry graphene electrodes and graphene as part of
the flexible wiring was realized and showed success-
ful ECG recording that is comparable to that of tradi-
tional wired gel electrodes.

For future applications, the development of all
printed electronic components such as transistors
and diodes will be crucial for enabling fully prin-
ted 2D material-based integrated circuits and sens-
ing platforms onto flexible and wearable substrates.
Within the Graphene Flagship project, strong pro-
gress has been achieved in both the production of
solution-processed 2D materials and their integration
into devices. However, many challenges still need to
be tackled [38]. For example, there is a limited num-
ber of p-doped semiconducting solution-processed
2D materials, and it is difficult to apply doping engin-
eering approaches because of the adsorbed residual
additives and solvent molecules onto the nanosheets.
Furthermore, in the case of solution-processed 2D
semiconductors, high performance in field effect
transistors is usually achieved by harsh treatments
with acids, which limit the number of substrates and
device architectures that can be exploited [38]. In gen-
eral, there is still little understanding on the funda-
mental properties of random networks of nanosheets
assembled into films, [60, 61] making difficult to
optimize device performance.

Furthermore, while most of the research effort
is focussed on improving device performance para-
meters, the dramatic increase in demand for pervas-
ive and versatile electronics also leads to concerns
regarding the sustainability of advanced materials,
ranging from ink production to product manufac-
turing and its end of life-cycle, such as the potential
impact of chemicals and materials used in inks on

human health and the environment. It is therefore
of crucial importance to consider sustainability issues
in the development and integration of 2D materials
into devices. In this framework, sustainable produc-
tion of graphene and use of life-cycle assessment are
starting to attract lot of attention in the community
[39]. Printing as an additive fabrication method has
considerable environmental benefits in comparison
to conventional PCB manufacturing process [62]. It
has been reported for printed smart labels that most
of the impacts result from silver paste production,
hence it is evident that replacing silver with a mater-
ial having lower environmental burden would con-
siderably reduce the impacts [63]. Furthermore, the
price of GRM plays an important role in the indus-
trial implementation. Schmaltz et al have carefully
analysed the current pricing of graphene. [64] They
report a wide range of current prices between $50
and $1000 per kilogram (kg), with the upper limit
at the same level as the price of silver. Schmaltz et al
also estimate a price decrease down to $12 per kg in
2028 (median price $40 dollars per kg), which could
be achieved with advancements in production meth-
ods. Hence due to anticipated sustainability and price
benefits, it is worth exploring GRM-based alternat-
ives even though their exploitation in e-textiles is cur-
rently restricted by high electrical resistance.

Data availability statement

All data that support the findings of this study are
included within the article (and any supplementary
files).

Acknowledgment

This project has received funding from the European
Union's Horizon 2020 research and innovation
programme under Grant Agreement No Numbers
785219 and 881603 referring to Graphene Flagship
GrapheneCore2 and 3, respectively.

ORCID iDs

Maria Smolander https://orcid.org/0000-0002-8335-4132

Laura S van Hazendonk https://orcid.org/0000-0002-3087-7191

Heiner Friedrich https://orcid.org/0000-0003-4582-0064

Tim Biemelt https://orcid.org/0009-0004-4987-1701

Khaled Parvez https://orcid.org/0000-0003-2851-9907

Marco Pelin https://orcid.org/0000-0002-4306-7411

Paolo Samorì https://orcid.org/0000-0001-6256-8281
Cinzia Casiraghi https://orcid.org/0000-0001-7185-0377

Henrik Sandberg https://orcid.org/0000-0002-2367-5639

Teemu Ruotsalainen https://orcid.org/0009-0008-1759-6273

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