Towards More Sustainable Interactive Textiles: A Literature Review on The Use of Biomaterials for eTextiles.

The development of functional fibres, active materials, and flexible electrical components has introduced new ways of embedding interactive capabilities within textiles. However, this seamless integration poses challenges in terms of materials, disassembly, and disposal, revealing an urgent need to address the issue of sustainability when creating new electronic textiles. Authors have proposed eco-design guidelines that emphasise the use of renewable and biodegradable materials. Despite these recommendations, the potential of biomaterials in eTextiles remains largely unexplored. This integrative literature review showcases how biomaterials emerged as catalysts for expanding possibilities within eTextiles and HCI, not only through their environmental sustainability but also through their dynamic and transformative nature, fostering a realm of novel interactive experiences. We suggest the potential of developing fully bio-based eTextile systems, the need for broader sustainability and aesthetic studies, the relevance of DIY methods, and the urgency of textile knowledge integration.


INTRODUCTION
Textiles with integrated electrical capabilities, i.e. electronic textiles (or eTextiles), are attracting increasing research interest as they represent a fexible, conformable platform for wearable and interactive applications.The potential application areas cover a broad spectrum of various use contexts, ranging from stroke rehabilitation [158] and tracking physical training performance [133] to smart home interfaces [60].Due to their versatile application areas, and how in combination with technological advancements in material science and fexible electronics they enable more comfortable electronics integration, eTextile products are also gaining increasing consumer interest.This has led to notable growth in the eTextile market, which is expected to reach US 1.4 billion by 2030 [59].As the eTextile market sits at the intersection of the textile and electronics sectors, the market infltrates and integrates aspects of both.Although it introduces an intriguing interdisciplinary playground for developing sophisticated wearable devices, this convergence also entails signifcant waste management challenges that both felds are facing [7,112].More specifcally, the integration of electrical components and textile materials introduces new additional challenges related to material resources and waste management [144], which are yet to be solved.Hence, the sustainability concerns stemming from eTextile manufacturing, utilisation, and disposal urgently need addressing, which calls for integrating biodesign guidelines and aiming for more sustainable options, such as prioritising renewable and recycled materials [76].
In practice, eTextile development involves a high degree of electrical component integration into textiles to increase the comfort, fexibility, robustness, reliability and maintenance of eTextile applications.This quest for seamless integration, and subsequently for better user acceptance [120], has led to a wide range of textile production methods.Additive textile fabrication methods such as lamination [145] have been used to attach circuits to readymade textile substrates.Lamination typically uses thermoplastic polyurethane (TPU) flm for bonding.Although it enables the fabrication of stretchable and reliable eTextile systems [145] and the disassembly of electronics for recycling, the approach includes several additional production phases, and uses more material resources.Electrically active yarns and fbres [87], fexible electronics [27], and yarns with embedded components [56,57] can also be integrated into textile structures during the textile substrate construction process, making them inseparable parts of the fabric.For instance, using silver-coated nylon yarns has led to the development of intricate multilayer woven sensors and circuitry [114], ofering possibilities to fully embed eTextile systems into woven textile structures.Sealing the electronics inside the textile protects the electrical system from external exposure, as well as the wearer from the system.This approach also allows the construction of eTextile systems that do not compromise the fabrics' visual and tactile aesthetic qualities, which is essential for product adoption [72].However, such an approach introduces several sustainability issues.First, such eTextiles combine raw textile materials and electronics crafted from scarce resources, heavy metals, and toxic chemicals [44].Second, as the electrical properties are embedded in the woven structure, the conductive yarns cannot be disassembled from the fabric, hampering its recyclability.Additionally, system-level integration may also involve the utilisation of potentially hazardous substances, such as fexible energy storage devices [124].
Augmenting textiles with electrical capabilities also has a significant impact on products' life cycles.Once in use, eTextile products are likely to be situated as mass consumer goods in the fashion and electronics sectors, promoting evolving styles and technological obsolescence [44], as well as cost-efective applications that are discarded rather than repaired.In addition, complex constructions, a combination of fabrication techniques such as weaving and soldering, and uncommon maintenance procedures reduce the repairability of such products.At the end-of-life stage, the intricate integration and diverse waste streams pose challenges to recyclability.As the eTextile market is still evolving, the industry sufers from a lack of standardisation, product traceability, and limited process feasibility.Currently, the WEEE Directive 2012/19/EU [28] regulates eWaste in Europe, while the OEKO-TEXr Standard 100 certifcation [131] ensures the elimination of harmful substances from textile supply chains.However, no specialised regulations exist for eTextiles.This absence of regulations has limited the deployment of adequate recyclability systems, and consequently, the recycling rate has remained low in Europe, where only a third of entities engaged in electronics and textile collection and recycling have established specialised processing strategies for eTextiles [34].Without processing strategies, removing and reusing electrical parts such as conductive yarns is unfeasible.As a result, the products are shredded into smaller pieces in the recycling stage without being separated into constituent components, and the scarce materials embedded in the eTextile systems cannot be salvaged [44].Thus, eTextile sustainability faces manifold challenges, which revolve around issues related to the union of electronics and textile materials, as well as to the life cycle of eTextile products.Reciprocally, addressing these challenges requires a holistic approach in which integrating life cycle thinking and circular economy models is pertinent [93].Several eTextile scholars have investigated these challenges by focusing on diferent aspects of products' life cycles and introducing eco-design guidelines [76] (Figure 2).
To start with the design and development phase, more attention must be paid to the users' needs.This could be achieved with a purpose-led electronics and textile system design process in which the textile design process is steered towards considering the meaningful use of the end applications [152].Involving users actively in the design process by employing co-design methodologies engenders user afnity, thereby enhancing product longevity [108].To this end, a multi-tiered approach comprising refective/expressive, behavioural/functional, and visceral/aesthetic dimensions has been suggested, to shape the framework for crafting sustainable smart textiles and clothing [44].During the manufacturing phase, more sustainable eTextile production could be approached through a systematic design framework that adopts 4R eTextile design principles involving repair, recycling, replacement, and reduction [125].In the use phase, sustainable consumption and product longevity can be achieved by creating lease-service systems [145], enabling repairability via modular solutions [56], establishing compatibility standards [76], avoiding functional obsolesce by promoting repairability [144], and advancing consumer awareness [108].On the later stage the end-of-life, proper labelling, modular design, and removable components can provide solutions for increasing the recyclability of eTextile products [76,145,156].Additionally, extending the scope of the WEEE Directive to cover eTextiles would contribute to legal frameworks for efcient recyclability [34].
Going back to the initial stage of material acquisition phase, the authors emphasise seeking alternatives sourced from renewable, non-toxic, ideally local, and biodegradable materials [52,76,124].This principle is exemplifed in Van der Velde's Life Cycle Assessment [142], in which material selection emerges as the most benefcial strategy for decreasing the environmental impact of smart textile products.Although rare, a few studies have explored the realm of green electronics for eTextiles and wearables, highlighting the utility of biodegradable metals, conductive polymers, organic semiconductors, graphene, starch, and gallium-based liquid metals [8,32,124,158].Nonetheless, sustainable material alternatives have not been extensively explored, despite their direct implications for manufacture, usage and disposal, and their potential to signifcantly mitigate the environmental impact.
To enable a better understanding of the options for sustainable eTextile production, this paper delves into a specifc category of materials with substantial promise as substitutes for the conventional plastics and metals ubiquitous in eTextile production: biomaterials.This review seeks to identify what and how biomaterials have been utilised in the creation of eTextiles.It also draws attention to how biomaterials can contribute to making human-material interaction applicable in eTextile development within human-computer interaction (HCI) research, and the extent to which these materials can contribute to more sustainable eTextile production.

BACKGROUND 2.1 Biomaterials
On the basis of the word's prefx, biomaterials can be understood as materials connected to living entities.In the feld of biomedical research, they refer to various synthetic and natural materials, such as polymers or ceramics, used in the treatment of any tissue, organ or function in the human body [134].In the domain of design, the interest in biomaterials is pushed by sustainability-driven trends, such as the war on plastics or veganism, together with the aim to incorporate living organisms into products [84], resulting in the linkage of material science, biology, design, arts, and crafts.Within this context, biomaterials are mainly understood as bio-based materials with a certain percentage of biological components [84].Depending on the literature source, the percentage required to fulfl this defnition varies from 10% upwards to 100% [84,129].This divergence infuences the level of biodegradability, which, according to Materiom [129], must be 100% guaranteed.Contributing to the ongoing defnition process, Weiss et al. [2002] introduce a territorial dimension to biomaterials.They assert that these materials should only be from local biomass and 'subscribe to the principles of circular economy and green chemistry, which nourish the territory in its decomposition process' [150].For the scope of this review, we consider biomaterials as materials that are 100% biobased and capable of complete biodegradation [130,150].They can originate from cultivating biological raw resources, such as mycelium, or from waste biomass, such as chitin.
The potential of biomaterials as better alternatives for product development stems from their role in future circular economies to help achieve some of the United Nations sustainable development goals [102].According to Rosenboom et al. [2022], the introduction of bioplastics, one of the largest categories in biomaterials, could contribute to 'diverting from fossil resources, introducing new recycling and degradation pathways, and using less toxic reagents and solvents in production processes'.Taking this into account, biomaterials and bioplastics are not, by default, more sustainable.Thus, life cycle assessments have to be conducted to determine the impact of every step, from feedstock harvesting to end-of-life [119].
In the case of textile and fashion industries, a frst generation of companies have developed biomaterials such as mycelium leatherlike sheets [100] or non-woven textile materials from pineapple leaf fbre [3]. Lee et al. [2020] have compiled a comprehensive list of specialised vocabulary and of these innovations [84].In the electronics feld, research has shown that biomaterials such as silk fbroin, cellulose and chitin can be used in fexible electronic devices in health monitors and biosensors [132].
Particularly in HCI, interest in biomaterials and integrating living organisms for eco-conscious biodigital interfaces is surging.This interest is framed, frstly, by a growing body of research on a material-driven approach that has contributed to a better understanding of how systems are crafted [38] and how the material properties of the things we interact with infuence user experiences [69].A second motivation stems from sustainability concerns within the HCI community.Researchers aim to incorporate recyclable and compostable materials into their prototyping processes, such as clay for 3D printing [19].They are also conducting life cycle assessment studies in digital fabrication [83] and connecting HCI practice to sustainable development goals [54].
So far, materials such as mycelium [46,80], food-waste bioclays [15], bacterial cellulose [106], iridescent bacteria [78], and bioluminescent algae [10,107] have sparked innovation in interaction interfaces in HCI.Various methodological approaches have been employed to examine material properties, characteristics, agency, and their relationship with the human body.Crafting and DIY techniques have been embraced to explore biomaterials in electronics [80], emphasising care, patience, and comprehension during design stages [16], and to transfer educational knowledge [20].Autoethnographic research has studied 'life as shared experience' and more-than-human design perspectives [78,106].These experiences have led the way to promoting organism-centred design and ethical considerations of the well-being of non-human organisms [107].
This review focuses on the search for and analysis of the available research on biomaterials applied in creating eTextile system elements by integrating studies framed in the feld of eTextiles and soft wearable electronics.

Biomaterial Defnitions
2.2.1 Renewable sources.Compared with non-renewable petroleumbased materials and metals commonly used in eTextiles, biomaterials originate from renewable sources, meaning they can be part of circular production processes.They can be obtained through the cultivation of biological raw material or the processing of waste biomass.'Biorefneries' convert renewable bio-based feedstock into useful chemicals, such as cellulose from straw or alginate from algae.In order to reduce their carbon footprint, their production should incorporate generative feedstocks derived from regenerative industries and processes [129], they should not compete with food production [119], and the manufacturing processes need to be energy efcient.

Biodegradability.
Biodegradation is the chemical transformation of a material carried out by microorganisms naturally found in the environment: during this process, these materials are converted into natural small substances such as water, carbon dioxide and biomass, without the need for artifcial additives.Importantly, the biodegradability of a material is determined by its chemical composition rather than its source: '100 percent biobased plastics may be non-biodegradable, and 100 percent fossil based plastics can biodegrade.'[18].The rate and success of biodegradation are infuenced by various factors, including environmental conditions (e.g.temperature and type of soil), the specifc material in question and its intended use; this is why it is important to specify the diference to composting, a biodegradation process that takes place under controlled conditions in a given time frame and can be carried out at home, in a laboratory or in an industrial plant.Thus, the main diference is that biodegradation is a property of a material, whereas composting is a specifc process [43].

2.2.3
Biocompatibility.This is the ability of a material, or substance, to generate a biological response in a determined situation, without producing harmful efects.It also refers to the compatibility of the substance with the living environment [153].In most cases, biomaterials can be innocuous and non-toxic for biological systems, particularly the human body.This characteristic makes them suitable for interactive textile products that need to be in close contact with the body, such as wearable devices or smart clothing [42].
2.2.5 Bioplastic.Currently used in industry, bioplastics are defned as bio-based, biodegradable, or both [18].This review will focus on the search and analysis of available research about biomaterials applied in creating eTextiles systems elements, integrating studies framed in the feld of eTextiles and soft wearable electronics.

METHOD
This integrative literature review [151] addresses the lack of a comprehensive overview of how biomaterials have been used in more sustainable eTextile research and development.Due to the multidisciplinary nature of biomaterial research, we gathered the literature included in this review from multiple outlets and various disciplines.Furthermore, to gain a deeper understanding of the researched materials, their use, benefts and challenges, the sources were analysed and discussed from two perspectives: eTextile design and material science.
In the frst phase, we searched leading journals and selected conference proceedings from the felds of textile design, HCI, electrical engineering, and material science (see Section 4.1 for more details).Our search keywords were 'electronic textiles' and 'bio', in the paper´s names or abstracts.The frst keyword excluded smart textiles with no electrical capability, together with wearable technologies and green electronics that did not include textile substrates.The word 'bio' gave us fexibility to fnd materials that were later fltered by 'bio-based' and 'biodegradable'.Materials that had only biodegradability traits, for example, were excluded.Due to the current extensive research on carbon and graphene fbres and textiles, an extra inclusion criterion was considered, particularly for carbon-based materials.Only papers with a clear statement of sustainability were included, for which we used the words 'sustainable', 'environmentally friendly', and 'green'.We complemented the search by conducting citation tracking on the included papers, using the snowballing technique [154].This resulted in 33 sources being included in the corpus.
For categorisation, we used a concept matrix to group and analyse the papers.The selected concepts corresponded to eTextile system components: power sources, conductive traces, substrates, microcontrollers, sensors, efectors, passive elements (resistors, capacitors, inductors) and cases (Figure 6).This enabled us to identify which kind of components biomaterials are mostly being used for and how they are integrated into these systems.Secondly, we analysed the materials on the basis of their origin, manufacturing methods, possible formats, combination with other materials, and biocompatibility.In addition, the diferent authors highlighted their interaction capabilities on the basis of the presented results.Finally, we searched for patterns of biodegradability, life cycle assessments, user studies and scale.

General Summary of Results
The fnal search covered a total of 33 publications.These sources were found in 19 diferent outlets following the search literature review protocol.As illustrated in Figure 3, the majority of the research contributions emerged through material science outlets, followed by those related to HCI, electrical engineering and lastly, textiles.Particularly in textiles, we observed a clear gap.Even though research on smart textiles and electronic textiles was easily available, only two papers that match the inclusion criteria were found in a search of fve leading textile journals and conference proceedings.Therefore, the following results are currently lacking more presence of knowledge originating from the textile feld.
A total of 9 diferent types of materials categories were found, with carbon-based materials being clearly predominant in the research outputs (Figure 4).This may be due to the extensive existing research on the capacity of graphene, carbon black (CB) and carbon nanotubes (CNT) to conduct electricity, a key feature of eTextiles.For the other eight materials we found only one to three publications, portraying a considerable research gap.During the search, conductive polymers such as PPy and PEDOT:PSS arose: their capacity to conduct electricity and biocompatibility has led to extensive studies for their use in electronic textiles and fexible electronics [138].Nevertheless, they are not included in this review as they do not come from renewable sources nor can they biodegrade.
All the publications showed material development on a Technology Readiness Level 1 [101], portraying basic observed principles and formulating initial technology concepts and applications.We explain the fabrication processes and material details for all the cases, and for around half of them, we present in-body examples and   user studies (Figure 5).From the sustainability perspective, all papers had a small statement declaring why they were better, 'greener' alternatives, four presented biodegradability studies [13,14,149] and two presented a schematic of their life cycle [82,143].
In regard to the eTextile components, sensors are the most widely developed elements, and were mentioned in 14 sources (Figure 6).The papers specifcally described sensors as suitable for measuring strain, moisture and pressure.The second most frequent purpose is to use biomaterials as conductive traces, either in yarn or textile format.However, signalling is only fabricated with carbon-based materials due to their electrical conductivity.Interestingly, power sources and supercapacitors (SC) were also demonstrated, contributing to more sustainable sources of energy generation and storage.Efectors such as light-transmitting materials and shapeand colour-changing textiles were also presented, together with RFID and antennae for wireless communication.Finally, substrates and cases were also shown as alternatives for commonly used plastics to support and store the previously mentioned components.Considering the perspective of eTextile systems, we found no examples for the use of biomaterial for eTextile microcontrollers.

Material Characteristics
The following section presents the found materials, providing information about their properties, formats, processes, and material combinations (Figure 7).

Carbon-based materials.
Carbon-based materials are a diverse class of substances primarily composed of carbon atoms.They are known for their exceptional properties, including high strength, electrical conductivity, thermal stability, and biocompatibility.The versatility and adaptability of these materials have driven innovations across various felds, such as electronics, material science, and bio-medicine [12,128].Particularly related to the feld of eTextiles, carbon-based materials have shown to be positively implemented in textiles in diferent ways [21-24, 33, 88, 99, 146].
Within the array of these materials, graphene oxide (GO) has been used to create conductive silk fabric with fre retardant properties through dry-coating method, which consists of coating the fabric directly with a reduced GO hydrosol [68].Spray-coating of GO onto facemasks has been used to create human respiration monitoring systems.The area coated with the conductive graphene works as a sensor, capturing the diferences in resistance whenever the shape of the mask changes [17].This type of strain sensor has also been developed by doing vacuum fltration of GO dispersion, where the cotton flter itself becomes the substrate of the sensor [118].Modifying the surface of a sample can also be done by dip-coating a textile substrate into a tank containing the GO [136].
Another way to produce electrically active materials is carbonisation, a pyrolytic process that aims to convert a biomass product into a highly carbonaceous material.Liang et al.
[2022] created supercapacitors by carbonising Bombyx mori silk.Molybdenum dioxide nanoparticles were used as a food additive for the silkworm.This resulted in modifed silk yarns, achieving mass specifc capacitance [89].Choi et al. [2023] proposed another carbonisation product: a 3D fabric carbonised textile structure aimed to enhance the piezoelectric properties of a sensor.The 3D structure was fabricated by knitting the carbonised cotton into polyester, and then attaching it to a copper pressure sensor using silver paste [26].
In their versatility, carbon-based materials can also be used as dyes and inks.Liang et al. [2022] produced hydrophilic, biocompatible, and washable electronic textiles by simply dyeing the fabric in a sericin-graphene dispersion.This coating was then used as a strain sensor [89].Akbari et al. [2016] fabricated textile-based antennae [1], by adding a graphene-based ink on the top of a cotton fabric substrate using the doctor blade technique [1,4].
Carbon-nanotubes (CNT) can also be used in the creation of the fabric sensors [61].The peculiarity of the process lies in the wrapping of CNT sheets around a yarn, either cotton or spandex, similarly to a spinning process.Then, to fabricate the biosensing electrodes, knitting, weaving and braiding were exploited [146].
Finally, carbon-based sweat wearables for energy harvesting have been developed using biofuel cells (BFV), and stored in an SC [92].
When working with carbon-based materials, however, it is important to keep in mind that one must work safely.The issue of their toxicity vs. biocompatibility is still under discussion and there are studies in the literature that lead to extremes of both possibilities [53].The toxicity or biocompatibility of this class of materials seems to be closely related to their modifcation, size and concentration inside the human body upon inhaling or ingestion.That is why it is still an open question and fruit of various researches even nowadays [90,97,111,113,123].
Regarding end-of-life, it is known that the biodegradation of carbon-based materials is difcult, but there are studies that continue to refne the process using enzymes.Liu et al. [2022] reported how, thanks to the digestive action of microorganisms and enzymes in yellow mealworms´ guts, it is possible to biodegrade GO sheets [91].In the case of Yang and Zhang [159], they focus their review on how the use of macrophages, white cells of the immune system specialised in the detection and elimination of harmful substances and bacteria, makes the biodegradation of diferent types of CNTs possible, pointing out that the time required for this process depends on diferent types of factors, such as their length and functional groups on their surface.

Cellulose.
Cellulose is a natural carbohydrate that serves as a primary structural component in the cell walls of plants and certain algae.It is one of the most abundant organic compounds on Earth and today plays a crucial role in industrial, technological and biomedical applications due to its mechanical properties, biocompatibility and easy modifability.Cellulose, the source of which is plants, usually undergoes treatments to be extracted and made usable (i.e.nanofbrils and nanocrystals).These treatments, for instance the dissolution or mechanical separation of the plant fbres, lead to the presence of functional groups along the polymer chains.
[2022] developed textile-based optical sensors using carboxymethylated cellulose (CMC).Two diferent types of CMC waveguides were achieved: fbres through wet spinning and planar by polymer casting.To create the optical sensors, both of the waveguides were weaved with either cotton or polyester yarns [50].Guridi et al. [2023] also tested diferent weaving patterns and changed the ingredients of the waveguides, adding glycerine to enhance the fexibility of the material [51].Wei et al. [2022] summarised the usage of cellulose as a 'green triboelectric generator'.It can be implemented in various devices, such as pressure sensors, smart home systems, and medical and environmental monitoring systems [149].
Another studied source of nanocellulose comes not from plants, but from bacteria: bacterial cellulose.Commonly known as kombucha, it is a pure, highly crystalline form of cellulose, the repeating unit of which is the same as that found in plant cell walls.However, due to its purity and crystallinity bacterial cellulose is unique in its properties and production process [6].Bell et al. [2023] have created an interactive breastplate using SCOBY (Symbiotic Culture of Bacteria and Yeast).SCOBY was grown in water, sugar and fnally tea for the colour, which was added to the started culture.When it reached the adequate dimension, it was given proper shape and dried.Different electronic components can be embedded in it, such as control systems, LEDs and sensing parts [14].Ng [2017] also worked with kombucha, similarly growing this bacterial nanocellulose, mixing it with dyes and embedding it with electronics [103].Cellulose is also very well known to be biocompatible and biodegradable, like the above cited materials, but its degradation conditions and time depend on its diferent types of functionalisation (e.g.cellulose acetate, cellulose xanthate), creating diferent possibilities [35].Hayakawa et al. [2014] have studied the efect of temperature on the rate of biodegradation, focusing on its burial in the forests of Japan, Indonesia and Thailand [58].

Polylactic acid (PLA).
Polylactic acid (PLA) is the biodegradable polymer obtained through the polycondensation of lactic acid, an organic acid that can be produced either via fermentation of, for example milk or whey, or chemical reactions.PLA is applied in a wide spectrum of felds, such as medical applications [122], agriculture [75], and automotive felds [105], and has recently attracted interest for the production of yarns for textiles [160].PLA is now widely used in 3D printing and studied for its shape memory properties [157].Leist et al. [2017] combined these two characteristics to create the 4D printing of PLA: a 3D model that can change shape when exposed to a heat source [86].They frst demonstrated how diferent shapes could be printed which, when compressed or stretched when hot, then cooled and brought back to 70 °C, permanently regain their original shape.This particular feature has been exploited to create a shape memory fabric in which nylon flaments are interwoven with 3D printed PLA.
Another example of the use of PLA for smart textiles is the work of Gong et al. [2022].Their process design enabled them to obtain hollow PLA fbres that were flled with eutectic gallium-indium alloys, then weaved into a fabric to create reconfgurable green electronic textiles [45].The main feature is that the two components, PLA and metal alloy, can be separated by a simple dissolution of the PLA fbres in dichloromethane, dissolving and recovering the gallium-indium alloy.Furthermore, Kalita et al. [2021] demonstrated the biodegradability of this material under diferent conditions and how it can be infuenced by adding algae as additives [71].

Mycelium.
Mycelium is the vegetative, thread-like structure of fungi, consisting of a network of branching hyphae.It serves as the foundational part of the fungal organism and plays a crucial role in various ecological, industrial and technological contexts [5].Like gelatine, mycelium, is often used as a stifer scafold to introduce or combine other functional materials or electric components.But, as a living system, the peculiarity of this natural material is that it can be grown directly in diferent desired shapes.Genç et al. [2022] have created diferent types of interactive samples.Crumbs of this fungus were placed inside polystyrene moulds and allowed to grow in desired shapes, and diferent components were added: LEDs for light-emitting devices, and heating plates for thermo-induced colour change or moving samples [41].Similarly, Vasquez and Vega [2019] used mycelium to fabricate 'Myco-accessories'.They also grew the material but used a lamination technique, sandwiching the electronic components between two thin layers of mycelium to create bracelets, crowns and other accessories.Mycelium is soaked into glycerine to confer more fexibility to the fnal piece [143].Being a living system, it biodegrades once buried in soil, even when part of a composite.Gan et al. [2022] demonstrated this by burying their mycelium and bamboo samples in soil for two months, after which only the bamboo parts remained [40].

Gelatin.
Gelatin is a versatile protein derived from the collagen found in the connective tissues, bones and skins of animals, primarily those of cows and pigs.It has been used for centuries in various culinary, pharmaceutical and industrial applications due to its unique gelling, stabilising and binding properties.Moreover, unlike collagen, which is composed of three chains twisted together, gelatine is linear and easier to modify [2].Gelatine, in combination with water and other components, can form strong, malleable or ductile foams, which are practical for extrusion or moulding to create desired shapes.Lazaro et al. [2022] created diferent types of interactive biofoam by exploring diferent fabrication techniques: moulding, layering, extrusion, and sewing.They produced colourful foams by adding dyes, conductive foam by adding steel fbres, and thermal and UV responsive accessories by adding photochromic or thermochromic pigments [82].The biodegradation of gelatine, which can be derived from diferent animal sources, depends mainly on its molecular weight [121].Martucci and Ruseckaite [2009] developed a three-layer based flm with modifed gelatine and compared its biodegradability with that of pure gelatine by burying both in soil: the latter showed a weight loss of 40% in one week [94].

Agar Agar.
Agar Agar, known also as Agar is a natural polysaccharide obtained from algae.Due to its special properties, it has been used for millennia in Asian cuisine and is widely known and used in many felds around the world.Agar agar is a versatile and valuable ingredient in the culinary and scientifc industries because of its ability to form gels when combined with water [109].Bell et al. [2022] explored the possibilities of their 'Alganyl', made simply by boiling agar together with glycerine and water; the mixture then casted and dried.Its re-cookability is an interesting aspect of this material, as it can be casted again for new purposes.As a flm, Alganyl has been tested in diferent ways, such as weaving, knitting, pleating, and braiding.It can be mixed with dyes, and combined with other active materials.Conductive Alganyl is obtained by adding activated charcoal paste.Photo-and thermochromic features have also been achieved by mixing it with photoactive and thermoactive dyes.It can also be used to create 3D shapes, such as a purse [13].
Some gram-positive bacteria possess an enzyme called agarase, which hydrolyses agarose, one of the components of agar agar.In their work, Parashar and Kumar [2018], used some of these bacteria for agar degradation.An interesting result is that temperature infuences the pH and action of agarase [110].

Chitosan.
Chitosan is a biodegradable and biocompatible polymer derived from chitin; a natural polymer found in the shells of crustaceans as well as in the cell walls of fungi.It is valued for its unique properties and has found a wide range of applications in various industries, such as agriculture, food, biomedics, and textiles [77].Frequently, due to its good mechanical and antibacterial properties, chitosan is used as a biocompatible scafold for making functional textiles.Tian et al. silver-coated polyamide with chitosan containing yarn through a spinning technique and weaved it into a cotton matrix.The resulting fabric as put in contact and separated with poly(dimethylsiloxane) (PDMS), which creates charges that are easily transportable by the silver.This results in a more eco-friendly tribogenerator [137].The use and implementation of chitosan is a good biodegradable alternative to other plastics.Wronska et al.
[2023] demonstrated this through the degradation of chitosan flms in soil.Chitosan swells when exposed to moisture and thus allows microorganisms to penetrate it, aiding its biodegradation, which takes place within a few weeks [155].

Escherichia coli.
Escherichia coli (E.coli) is a species of Gramnegative bacteria present in both human and animal intestines.E. coli is a major topic of microbiological research and is used in eforts to improve public health because, while many strains are safe and helpful for digestion, others can infect people and lead to sickness [74].Wang et al. [2017] focused on its application in humidity actuators by printing E. coli's cell suspension onto cis-1,4-polyisoprene (latex) flms.Moreover, through genetic engineering, they demonstrated the possibility of adding even new functionalities, in their case, fuorescence ability.Their work led to the creation of E. colibased sweat-responsive wearables [148].As a carbon-based living organism, E. coli eventually dies and biodegrades naturally.However, we found no papers on its actual biodegradation, probably because of a lack of interest in this feld.Bacillus subtilis is a Grampositive, rod-shaped bacterium that is widely recognised for its importance in various felds, including biology, biotechnology and agriculture.This bacterium has attracted attention for its unique characteristics and versatility, such as easy genetic manipulation and large-scale production [162].Similarly to E. coli, it has been studied as an actuator because of its humidity-responsive properties.However, it has not been implemented in textile structures yet [161].Also like E. coli, this living organism dies after some years and biodegrades naturally.4.2.9Qasi-solid ionic conductors.Quasi-solid ionic conductors are a class of materials that exhibit both solid and liquid-like properties.These materials have attracted signifcant attention in various felds, especially as electrochemical devices and sensors, due to their ability to transport ions efectively while maintaining mechanical stability.Quasi-solid ionic conductors bridge the gap between traditional solid-state electrolytes and liquid electrolytes, ofering unique advantages.The synthesis of quasi-solid ionic conductors can vary depending on the specifc materials and applications involved.These materials often involve combining a polymeric or macromolecular matrix with a suitable ionic conductor, for example, electrolytes.Lei and Wu [2021] have described the potential of these materials.The dough created by mixing the polymer matrix with diferent salts is malleable and ductile.In their review and work they show that the addition of the salt improves the mechanical properties as well as the conductivity of these materials.It is even possible to create bio-based and edible dough, the application of which relies on the creation of smart skin, which acts as a mechanical sensor; this is possible by creating two layers of ionic conductive dough and separating them with an insulating material (e.g.insulating double-sided tape).The dough can even be 3D-printed or spun to create more unique structures [85].
Being made of gluten and starch, and being a protein and a polysaccharide, this dough can easily biodegrade.In their study, Domenek et al. [2004] tested the biodegradation of diferent glutenbased materials, showing that when buried, gluten takes around 50 days to completely degrade [29].Furthermore, Jayasekara et al. [2003] demonstrated that in their starch-based flm, all the starch degraded in 45 days, utilising an automated composting unit [67].

Interaction Possibilities
Considering that eTextiles are meant for interaction, 20 publications clearly described how these human-biomaterial interactions could take place.The uses suggested included measuring external stimuli, physical activity, touch and pressure, or illuminating output elements (Figure 8 and 9).With one exception [86], the enabling eTextile components directly interact with the human body.Aligned with wearables, on-body placement is favoured, particularly on the neck, fngers, wrist, and forearm for motion-tracking prototypes (Figure 10).Positioned on the body, sample dimensions range from 2 to 20 cm [148], but only one paper describes an ambient display application [44].
To clarify their working principles, we present two categories: (1) Renewable and biodegradable biomaterials with similar properties to those found in commonly used eTextile materials (e.g.electrical conductivity), which enable interactions often encountered with smart textile interfaces.(2) Renewable and biodegradable biomaterials with new, different properties to those found in commonly used eTextile materials (e.g.hygromorphic), which enable new interactions with smart textile interfaces.
In the frst case, carbon-based materials are predominantly employed to imbue textile substrates with electrical conductivity.The resulting fbres and textiles are then incorporated into conventional applications, such as bending sensors [26,68,89,118,137], strain sensors [118] or antennae [146].Due to their renewable origin and capacity to biodegrade they ofer a more sustainable option, as well as enabling already known applications and interactions.
In the latter case, biomaterials ofer a more environmentally sustainable alternative, and also enhance potential interactions within eTextile systems through their intrinsic properties.Active materials such as cellulose, PLA, bacteria, and agar agar dynamically respond to environmental changes, facilitating shape deformations in response to humidity [86,148] or variations in optical qualities [50].These transformative attributes distinguish them from conventional eTextile materials and enable novel interactions such as the ability to perceive haptic changes through bacteria moving wearables [148].In addition, mycelium, agar agar and bacterial cellulose are capable of 'growing' and encapsulating diverse functional components, such as thermochromic inks or microcontrollers, creating new mediating interfaces between functional materials and the human body.
Notably, these bio-based interfaces introduce distinctive characteristics in comparison to the traditional use of plastics or metals in eTextiles.The tangible qualities such as agar agar´s transparency [13], the natural feeling of mycelium [41], and the soft and fexible texture of gelatine [82] introduce interesting tactile interaction possibilities, such as the ability to press or squeeze or establish an intimate connection between a 'lifelike' bacterial cellulose and the human body [14].
Conclusively, their versatile manipulation capabilities, including coating, foaming, moulding, cutting, weaving, or knitting (Table 7), underscore the potential of these materials to craft a diverse array of eTextile systems -ranging from necklaces and headbands to breastplates.Again, this amplifes the potential for curated interactions among functional materials, the environment, and the human body.
In order to better understand the potential of the new interactions enabled by these biomaterials, authors [13,14,41] delved into the experiential qualities of biomaterial-based eTextiles.Their main insights highlight that in comparison to traditional eTextile materials, biomaterials can be perceive as fragile, provoke a feeling of 'uncanniness' or 'creepiness' [41].They can also provide sensory-rich experiences [82], foster a sense of 'shared livingness' between the human wearer and the non-human organism [14], and present positively charged characteristics such as biodegradability, fexibility, and ease of fabrication [13].

INSIGHTS FROM THE LITERATURE
The preceding results section highlighted 9 biomaterials identifed in contemporary research for constructing components within eTextile systems.It presented their key attributes, including biodegradability and biocompatibility, along with their diverse array of applications and formats ( Figures 7,8,9).A closer examination of the literature unveiled a promising avenue for developing sustainable eTextiles, in which the inherent qualities of bio-based materials enable novel interactions.These materials can actively respond to the environment, integrate functional components, undergo biodegradation, and be manipulated through various techniques such as weaving, knitting, printing, moulding, growing, pleating, cutting, or sewing.The next section presents an in-depth analysis of the results, and ofers seven primary insights: (1) Biomaterials for eTextiles are an under-explored feld (2) Fully bio-based eTextile systems can be constructed (3) Biodegradability is a design variable (4) Even though promising, the environmental impact of biomaterials for eTextiles remains unclear (5) DIY approaches are contributing to material innovation in the interdisciplinary feld of eTextiles (6) Biomaterials provide an opportunity to amplify the interactive possibilities of eTextiles (7) The aesthetics and interpretative understandings of biomaterials for eTextiles should be deepened   The frst notable issue emerging from the literature is a clear research gap in the use of biomaterials for eTextiles.We found only 33 publications in 19 diferent outlets, including various journals and conferences.
Half of these sources revolve around carbon-based materials, a well-established domain within novel conductive materials research.Their lightweight, robust, and conductive properties render them ideal for crafting various components within eTextile systems, including sensors and antennae.Nevertheless, the number of publications explicitly addressing the reasons and methodologies for utilising these materials as more sustainable alternatives is notably insufcient in comparison to the overall fndings.The other half of the results comprise height distinct material categories: mycelium, cellulose, gelatine, salts, agar agar, chitosan, PLA, and quasi-solid ionic conductors.Each category is underrepresented, with only one to three papers discovered for each one, all published between 2017 and 2023.This scarcity highlights that this research area is still in its initial stages.
Regarding knowledge clusters, material science is predominant, followed by HCI and electrical engineering.In contrast, textile design has minimal representation, and we found only two papers on it.This supposes a relevant research gap to be addressed by future research.Knowledge steaming from textile research and practice is fundamental to enrich the feld of sustainable eTextiles.Integrating deeper understandings on textile thinking theory [62], fabrication methods that could enhance functionalities [9,115], or circularity for fashion and textiles [104,141], among others, will contribute to more holistic and purposeful developments of biobased eTextiles.Additionally, insights from the felds of green electronics [132] and industrial design [70], should be integrated through dedicated literature reviews of biomaterials' applications within these domains.It is relevant that these contributions stem from interdisciplinary teams, as their diverse methodologies, resources and epistemologies could enrich the fndings.Collaboration should be actively encouraged through initiatives such as the Bioinnovation Center [140], Software [126], STARTS Programme [117], or Bio-Inspired Textiles [63].

Fully Bio-based eTextiles Systems Can Be Constructed
Understanding eTextiles as systems highlighted the versatility of biomaterials for creating diferent components, taking into account not only conductive elements but also those that can contain, bind, protect, react, or store.The catalogue of biomaterials we found presents an array of diferent formats and processes, each ofering multiple possibilities, as shown in Figure 7. Carbon-based materials such as inks, flaments, pastes and liquid solutions showed potential to create various elements: power sources, conductive traces, sensors, efectors, passive elements, antennae, and RFID tags.In addition, the use of conductive salts in dough shapes enable the creation of fexible and malleable sensors.Mycelium and gelatine exhibit great potential for encapsulating other materials, thereby enhancing their capabilities as sensors and efectors.Simultaneously, they take on the form of malleable 3D shapes, serving as both substrates and protective casings.Agar agar, presented in its flm confguration, can undergo diverse transformations through techniques such as heat sealing, weaving, knitting, pleating, braiding, and laser engraving, leading to the creation of colourful, fexible substrates.In the case of Escherichia coli, living bacteria enabled textile movement, avoiding the need for metallic or plastic fbres.Lastly, cellulose, whether derived from plants or bacteria, proved to be useful for power sources, sensors, efectors, and substrates.This is particularly interesting due to the growing interest in green electronics and the use of cellulose for a wide range of components such as substrates for fexible electronics [66] or strain sensors [11].
The integration of these biomaterials ofers boundless opportunities to replace nearly all components in an eTextile system.Components made from diferent biomaterials could be combined in the same systems, or go even further and propose mono-material systems (e.g. with cellulose).No examples of bio-based microcontrollers were found in the literature, but inspiration could be taken from non-traditional computer constructions such as the 8-bit embroidery computer made by Posh and Kurbak [2018] [64].This potential could be further magnifed by embracing modularity [55] and disassembly [39,156] as integral design principles, facilitating material separation and component repair at the end-of-life stages.Consequently, we envision biomaterials as central agents in the realisation of fully bio-based eTextile systems, moving toward sustainable and eco-friendly textile technology.

Biodegradability Is a Design Variable
From the perspective of design, we considered biomaterials as coming from renewable biomass origins and/or as having the capacity to biodegrade.As mentioned in the defnitions section, biodegradability means that the materials can be degraded through microorganisms available in the environment, converting them into natural substances such as water, carbon dioxide and composts.This characteristic can greatly contribute to the end-of-life of eTextile products as part of circular processes in which materials can return to the soil without throwbacks.To determine the actual impact of the biodegradability and composting of the materials, collection and disposal systems need to be taken into consideration.In some cases, the composting of biomaterials can bring more challenges, due to the lack of special facilities for textile-related biomaterials, manipulation processes and the release of toxic gases [119].In the reviewed literature, biodegradation appears to be one of the main material benefcial characteristics.The agar agar user's study revealed that biodegradability was the most interesting aspect for the participants, inspiring them to imagine future uses of Alganyl [13].Three publications described biodegradability studies.One prototype of the cellulose-based energy storage element [127] was dug up and only 30% of Vim's mass remained after 60 days.Bell et al. [2023] buried a 5 cm by 5 cm SCOBY sample in soil containing living microbes, which degraded by 96% in 30 days [14].In the case of agar agar [13], a paper claimed that Alganyl degraded by 97% in 60 days in a controlled environment kept at 40° C.
Biodegradability is thus becoming an important variable to consider when designing eTextile systems, as it can contribute a closer loop, either by returning to the soil, enabling disassembly or reutilisation.In addition, we see how degradability can also contribute to the material aesthetics and afordances of interactive systems.The time transformation of the material, due to interaction with people and the environment, could produce shape-or colour-shifting materials, opening an exploration path for novel interactions, as for example, in the artistic work of 'Cambio de Piel' [48], in which a text is revealed on a fabric layer as the top biomaterial layer dissolves in water.In the case of wearables design, these properties can also contribute to customisation and the exploration of ephemeral fashion [81].This changeability approach has been proposed by Talman [2019], and ofers new ways of exploring transformations in colour, texture and structure in an aim to understand how we can encourage acceptance of changes in textiles and re-establish textile-people connections [135].

Environmental Impact of Biomaterials for eTextiles Remains Unclear
The results presented ofer an inspiring justifcation of the exploration of more sustainable options in eTextile creation, driven by the advantages of biomaterials -their renewability, biodegradability, and, in most cases, biocompatibility.However, this is still a young research area, with all the material outcomes falling under Technology Readiness Level 1 [101].At this stage, the research primarily revolves around establishing fundamental principles and formulating initial technology concepts and applications.Out of the identifed publications, only fve presented a biodegradability study [13,14,99], and two presented a life cycle schematic [82,143].None of them presented a comprehensive life cycle assessment.This indicates the lack of clarity regarding the environmental impact of these materials.Ideally, a circular perspective should be ingrained from TRL1 onwards, as advocated by Schinchke et al.
[2020].It is crucial to continually refect on the possibilities and challenges associated with scaling up the production of these materials, including their infuence and interconnections with various factors of the life cycle [142], such as transportation, design, usage, and end-of-life considerations.Interdisciplinary collaboration is crucial in this objective, as it facilitates an inclusive analysis that incorporates laboratory-based investigations alongside social and economic factors from a systemic perspective [31].Critical perspectives are vital for examining production levels, material origins, treatment processes, and the involvement of communities within their ecosystems [150].For insights into addressing production challenges and understanding their implications on a larger scale, business experiences from eTextile [125] and biomaterials such as MycoWorks [100] can be instrumental.Lastly, staying attuned to the evolving legislation concerning Bioplastics and Biomaterials [36] will also provide guidance for future directions and unveil potential investment opportunities in this feld.

DIY Approaches Contributing to Material Innovation
We identifed a total of 30 distinct manufacturing processes, with the possibility of applying multiple methods to each material, as detailed in Table 7. Carbon-based materials often necessitate specialised laboratory facilities and equipment, such as vacuum fltration [118] and carbonisation [26].Scientifc research frequently uses nanoparticles that macroscopically result in a powder, requiring the usage of proper safety gear.More user-friendly suspensions are available for commercial applications, lessening the risk of safety hazards.Despite their format, some of these materials are often chemically modifed to improve their chemical or physical properties, depending on the application; one common example is the chemical reduction of GO to enhance its electrical properties [65].For E. coli [148], manipulation took place in special biolabs under a controlled environment that guaranteed biosafety, and had to obtain security approval.Even if naturally present in our intestines, some strains of E. coli can cause severe infections.In contrast, materials such as mycelium, gelatine, cellulose, chitosan, agar agar, and various salts are more accessible for purchase and easier to manipulate.The associated processes are closely related to cooking and textile techniques, eliminating the need for specialised laboratory facilities.They can be executed in a do-it-yourself (DIY) manner utilising open-source resources from various repositories and educational programmes such as Materiom [95], Fabricademy [37], Healthy Materials Lab [79], Chemarts [70], and Biodesign Challenge [25].This combination of DIY techniques and resource networks enables non-scientists to more freely prototype and try diferent fabrication such as mould casting [13], weaving [51] or laser cutting [143].Modifying moulds, combining materials, and altering scale, colours, or textures further enables the discovery of new material behaviours.
The literature review also highlights the potential for these materials to serve as playgrounds for afordance studies and collaborative workshops, inviting non-experts to engage in material manipulation and ideation processes [13].Therefore, we see how these material 'playgrounds' [16] can open the path for innovative processes and material combinations, which can be guided through design methods such as fast prototyping and material-driven design frameworks [73].This can be particularly valuable in the case of interdisciplinary collaboration, allowing designers to lower the entry barriers to understanding new materials through hands-on work, and for scientists to approach the material in diferent settings, new scales and from a more open-ended ideation process.Designers beneft by gaining a practical understanding of new materials through hands-on work, and scientists get to explore materials in diverse settings, at diferent scales, and through more open-ended ideation processes [13,116].In conclusion, these DIY approaches ofer the potential to discover innovative processes and material combinations, simplifying the integration of new materials into design practices and encouraging more versatile scientifc exploration.

Amplifying Interaction Posibilities
The analysis of interactivity (Section 4.3) revealed that biomaterials can signifcantly contribute to advancing current developments in soft interactive interfaces within HCI.They can, for example, provide carbon-based conductive yarns for smart sanitary napkins [98] in order to contribute to their disposal and biodegradability challenges, or present new bio-based tactile qualities to keep exploring afordances of surface gestures on textile user interfaces [96].
These biomaterials not only serve as substitutes for traditional materials but also enhance the interactive capabilities of eTextile systems in HCI.Beyond ofering a more sustainable alternative to conventional plastics and metals, these materials introduce novel properties that foster the exploration of new relationships among humans, the environment and interfaces.The diverse textures, such as rugged mycelium or adhesive biofoam, alongside various formats such as cellulose flms or PLA volumes provide researchers in the HCI feld with myriad options for experimenting with sustainable materials, thereby supporting the creation of multisensorial interactive experiences.A distinctive feature of biomaterials lies in their being both biodegradable and active materials, making them unique in their transformative capacity.These materials can deform, distort, shrink, swell, or degrade over time and in response to diferent environmental stimuli, introducing temporality as a new design variable [139].This characteristic brings forth the concepts of 'growth' and 'liveness, ' contributing to ongoing discussions on more-than-human entities and their agency in design processes and interactions.
An additional insight obtained from the literature pertains to the scale of eTextile examples predominantly situated on the human body in direct contact with the skin.This inclination can be attributed to the intrinsic relevance of eTextiles within the wearable technology domain, to which the majority of applications are targeted.To maintain this on-body focus, the scale of material samples and prototypes spans a spectrum ranging from a mere 1 cm in the case of conductive yarns and prints [30] to approximately 30 cm for the humidity-reactive shirt [148].As an exception, the mycelium study [41] presents a unique potential for application in interior design elements and ambient displays.We acknowledge the relevance and importance of small-scale, but we also see an unexplored feld of larger bio-based interactive interfaces.
Given the versatility of biomaterial production techniques and their accessibility, we advocate for the expansion of sample dimensions.As biomaterial-based eTextiles do not use scarce resources, heavy metals or toxic chemicals, which urge minimising electrically functional material consumption, this opens up the exciting prospect of creating larger interactive interfaces that may not necessarily adhere to the body but can facilitate novel HCIs through these materials.Examples of larger-scale interaction can be found in interaction design, such as electrostatic interior textiles [47], which involve room-size textile installations designed for triboelectric energy generation capable of shaping our everyday behaviours.As the electrostatic textiles did not specifcally focus on biomaterials, research on piezoelectric biomaterials [147] could provide directions for further research.Furthermore, for more extensive biomaterial surfaces, 1.5-metre tapestry pieces of hand-woven cellulose have been crafted for an interactive installation called 'Borrowed Matter' [49].The potential for upscaling material interfaces broadens the horizon for novel applications, exposing them to wider audiences and fostering collaboration with artists, architects, stage designers, performers, and construction engineers.These explorations could expand the creative landscape of the feld of human-computer interaction.
In summary, biomaterials emerged as catalysts for expanding possibilities within HCI, not only through their environmental sustainability but also through their dynamic and transformative nature, fostering a realm of novel interactive experiences.

Deepening Aesthetics and Interpretative Understandings
Understanding how users attribute value to specifc eTextile products holds the potential to foster sustainable consumption habits and encourage long-lasting usage [108].In this context, the aesthetics of materials assume an important role in shaping users' perceptions and experiences of interactive systems.In this regard, fve publications [13,14,41,51,82] explicitly manifested a keen interest in the aesthetics of their prototypes.Fiona Bell et al. [2023] shared their personal refections regarding their immersive experience with the SCOBY breastplate, simultaneously acknowledging and embracing the material's 'imperfections' [14].The natural colours and organic textures were regarded as personal and connecting elements that forged a bond between the practitioners and the material.Meanwhile, Guridi et al. [2023], in their creation of cellulose-based optical sensors, detailed their decision-making process concerning complementary materials and colour palettes [51].They accompanied this with comprehensive visual documentation aimed at enriching the evocative visual narratives surrounding the material.Extending this comprehension, user studies were presented in the cases of mycelium [41] and agar agar [13].In the frst case, the quest for experiential qualities led researchers to ask for participant feedback on whether the dynamic changes in mycelium samples were 'aesthetically pleasing' and to explore the meanings and emotions elicited by the material.These fndings revealed ideas on future applications and design directions for interactive mycelium products.In the case of agar agar, a user study served as a lens through which to analyse the material's desirable attributes and drawbacks, thereby identifying prospective applications [13].The authors also extensively explored colours, shapes and textures.We observe that integrating these qualitative studies into material development could inspire practitioners and researchers to explore new possibilities, harnessing user experiences and interpretations to craft more meaningful and purpose-driven eTextile products [152].Material-driven design methodologies [73] ofer a framework for conducting these studies and facilitating the efective communication of results within interdisciplinary teams.

CONCLUSION
Our comprehensive literature review on Biomaterials for eTextiles highlighted sustainable options for developing interactive textile interfaces.Despite the limited existing research, this efort synthesised insights from diferent felds such as HCI, Material Science, Electrical Engineering, and Textiles.To further advance this research, we suggest incorporating more knowledge steaming from Textile research, Green Electronics, and Design, even if not directly eTextile-related.
The fndings unveil promising directions, including the potential creation of fully bio-based eTextile systems.In this context, biomaterials emerge as increasingly sustainable alternatives, simultaneously enhancing interaction possibilities due to their distinctive characteristics.To comprehensively grasp their potential, integrating thorough life cycle assessments is imperative for evaluating their authentic environmental footprint.
Finally, we claim that these advancements necessitate interdisciplinary collaboration, integrating scientifc methodologies, user studies, eco-design guidelines, and aesthetic considerations.Such a holistic approach is key to unveiling the full potential of biomaterials in crafting sustainable and innovative eTextile solutions.

Figure 2 :
Figure 2: Diferent stages of life cycle thinking for circularity in eTextiles

Figure 3 :
Figure 3: Distribution of articles over diferent publication outlets.In total, 19 results where found in the feld of Material Science (dark red), 8 in HCI (red), 6 in Electrical Engineering (orange) and 2 in Textile related outlets (yellow).

Figure 4 :
Figure 4: Material categories found with their corresponding amount of publications.Carbon-based materials are the most studied ones for the development of eTextiles.

Figure 5 :
Figure 5: Publications showing on the body examples, user studies and biodegradability tests.

Figure 7 :
Figure 7: Summary of biomaterials with their origin, main properties, formats, manipulation processes and combination with other materials.

Figure 8 :
Figure 8: Schematic representations of the materials, their formats and interaction goals.Orange colour represents the main biomaterials, and grey colour represents the supporting materials.

Figure 9 :
Figure 9: Schematic representations of the materials, their formats and interaction goals.Orange colour represents the main biomaterials, and grey colour represents the supporting materials.

Figure 10 :
Figure 10: Placing of material samples observed in the literature