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

2.1 Biomaterials

On the basis of the word’s prefix, biomaterials can be understood as materials connected to living entities. In the field 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 fulfil this definition varies from 10% upwards to 100% [84, 129]. This divergence influences the level of biodegradability, which, according to Materiom [129], must be 100% guaranteed. Contributing to the ongoing definition 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 first generation of companies have developed biomaterials such as mycelium leather-like sheets [100] or non-woven textile materials from pineapple leaf fibre [3]. Lee et al. [2020] have compiled a comprehensive list of specialised vocabulary and of these innovations [84]. In the electronics field, research has shown that biomaterials such as silk fibroin, cellulose and chitin can be used in flexible 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, firstly, 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 influence 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 field of eTextiles and soft wearable electronics.

2.2 Biomaterial Definitions

4.1 General Summary of Results

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The final search covered a total of 33 publications. These sources were found in 19 different 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 five leading textile journals and conference proceedings. Therefore, the following results are currently lacking more presence of knowledge originating from the textile field.

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A total of 9 different 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 flexible 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].

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In regard to the eTextile components, sensors are the most widely developed elements, and were mentioned in 14 sources (Figure 6). The papers specifically 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. Effectors such as light-transmitting materials and shape- and 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.

4.2 Material Characteristics

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

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4.3 Interaction Possibilities

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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, fingers, 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:

In the first case, carbon-based materials are predominantly employed to imbue textile substrates with electrical conductivity. The resulting fibres 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 offer a more sustainable option, as well as enabling already known applications and interactions.

In the latter case, biomaterials offer 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 flexible 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 amplifies 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, flexibility, and ease of fabrication [13].

5.1 Biomaterials for eTextiles Are an Under-explored Field

The first 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 different 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 insufficient in comparison to the overall findings. 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 field 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 fields 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 findings. Collaboration should be actively encouraged through initiatives such as the Bioinnovation Center [140], Software [126], STARTS Programme [117], or Bio-Inspired Textiles [63].

5.2 Fully Bio-based eTextiles Systems Can Be Constructed

Understanding eTextiles as systems highlighted the versatility of biomaterials for creating different 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 different formats and processes, each offering multiple possibilities, as shown in Figure 7. Carbon-based materials such as inks, filaments, pastes and liquid solutions showed potential to create various elements: power sources, conductive traces, sensors, effectors, passive elements, antennae, and RFID tags. In addition, the use of conductive salts in dough shapes enable the creation of flexible and malleable sensors. Mycelium and gelatine exhibit great potential for encapsulating other materials, thereby enhancing their capabilities as sensors and effectors. Simultaneously, they take on the form of malleable 3D shapes, serving as both substrates and protective casings. Agar agar, presented in its film configuration, can undergo diverse transformations through techniques such as heat sealing, weaving, knitting, pleating, braiding, and laser engraving, leading to the creation of colourful, flexible substrates. In the case of Escherichia coli, living bacteria enabled textile movement, avoiding the need for metallic or plastic fibres. Lastly, cellulose, whether derived from plants or bacteria, proved to be useful for power sources, sensors, effectors, 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 flexible electronics [66] or strain sensors [11].

The integration of these biomaterials offers boundless opportunities to replace nearly all components in an eTextile system. Components made from different 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 magnified 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.

5.3 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 definitions 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 beneficial 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 affordances 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 offers 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].

5.4 Environmental Impact of Biomaterials for eTextiles Remains Unclear

The results presented offer an inspiring justification 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 identified publications, only five 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 reflect on the possibilities and challenges associated with scaling up the production of these materials, including their influence 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 field.

5.5 DIY Approaches Contributing to Material Innovation

We identified 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 filtration [118] and carbonisation [26]. Scientific 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 modified 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 different 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 affordance 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 different settings, new scales and from a more open-ended ideation process. Designers benefit by gaining a practical understanding of new materials through hands-on work, and scientists get to explore materials in diverse settings, at different scales, and through more open-ended ideation processes [13, 116]. In conclusion, these DIY approaches offer the potential to discover innovative processes and material combinations, simplifying the integration of new materials into design practices and encouraging more versatile scientific exploration.

5.6 Amplifying Interaction Posibilities

The analysis of interactivity (Section 4.3) revealed that biomaterials can significantly 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 affordances 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 offering 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 films or PLA volumes provide researchers in the HCI field 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 different 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 field 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 specifically 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 field 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.

5.7 Deepening Aesthetics and Interpretative Understandings

Understanding how users attribute value to specific 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, five 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 reflections 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 first 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 findings 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] offer a framework for conducting these studies and facilitating the effective communication of results within interdisciplinary teams.