Bio-based materials: an alternative to synthetic polymers for the development of lab-on-a-chip devices

Lab-on-a-chip (LoC) devices have proved their potential for biomedical and analytical applications. Despite their growing demand, their environmental impacts remain insufficiently taken into account. These microfluidic devices are mainly made from petroleum-based materials like PDMS and thermoplastics, whose life-cycle (from resource extraction to disposal) poses ecological and health concerns. In response, a growing number of research papers are exploring bio-based alternatives, such as cellulose, PLA, chitosan, or zein. This review details properties of microfluidic devices made from these bio-based materials compared with more conventional materials, and particularly their impact on the environment from raw material sourcing through manufacturing to disposal. Although many of these materials are still in early stages of research, and published data is limited, developments are promising, and the associated technological challenges must be met. The alternative to plastics used for the manufacture of LoC could be a panel of complementary bio-based materials, locally sourced to support the local economy and limit transport, and which do not lead to new imbalances. This review advocates for a sustainable approach to material selection, encouraging the development of greener microfluidic devices.

1 Introduction

The environmental crisis is compelling us to change our practices to reduce the greenhouse gases generated, the intensive exploitation of resources, the plastic waste and the resulting destruction of ecosystems, among other things. These issues become even more crucial for devices with very short usage time, as manufacturing constitutes by far the main source of pollution and energy consumption (UN Environment Program, 2021). This is particularly evident in analytical testing, which requires large quantities of consumables such as pipette tips and tubes, as well as considerable amounts of reagents (some of them being highly toxic). Downscaling analysis processes can reduce their environmental impact by reducing the materials, the reagents and solvents used, as well as the waste generated (Agrawal et al., 2021; Ma and Xu, 2023). In this context, Lab-on-a-chip (LoC) devices, which process and handle small quantities of liquids (a few pL to a few µL) in microchannels with sizes ranging from a few micrometers to a few millimeters, should lower the environmental footprint of analyses. Such devices yield an overall size footprint from a few cm² to a few dozen cm² and, by integrating several functions into a single device, they reduce the number of handling devices and the quantities of consumables used. The reduced consumption of reagents and shorter analysis times also lower the costs, while the portability of LoCs allows for analyses to be performed outside centralized facilities. Hence, LoCs have attractive properties that enable them to be used in a wide range of applications from biomedical applications (Dittrich and Manz, 2006) to contaminant detection for the food industry (Sridhar et al., 2021) or in the environment (Büyüktiryaki et al., 2019). As a consequence, this leads to an increasing demand for miniaturized, cost-effective LoC solutions, e.g., in Point-of-Care testing, in-vitro diagnostics, and drug discovery. This is demonstrated by the global market size of LoCs reaching USD 22.78 billion in 2024 and being expected to reach UDS 54.61 billion by 2032 (Newswire, 2025). This fast-growing market makes it imperative to conduct more precise evaluations of the environmental impacts of LoCs, since these devices are mostly fabricated from petroleum-based materials and are often dedicated to single use. Hence, it has become crucial for researchers and engineers to investigate new solutions to improve the sustainability of LoC technologies.

A valuable tool in the search for sustainable solutions is Life Cycle Assessment (LCA), as it enables to identify the key steps with high environmental impact and to make informed decisions to reduce the overall environmental footprint of a product. As illustrated in Figure 1, LCA considers all the environmental impacts across all stages of a product’s life cycle, including raw material extraction, production, distribution, use, and disposal. By evaluating the consumptions in terms of raw materials, water and energy, as well as the emissions, the waste and their respective environmental impacts at each stage, LCA can guide towards more sustainable material, technology and design choices. However, the specific use of the device and the related regulations also have to be considered when investigating new solutions. Due to the requirements for safe disposal of infectious waste and to avoid cross-contamination, most LoCs are currently neither re-used nor recycled, although this could considerably lower the environmental impact of the devices. According to Ongaro et al. (Ongaro et al., 2022), the waste management of LoCs in medical settings leads to either their incineration or disposal in landfills, which means that the device’s end of life should play a crucial role in the choice of materials that could be proposed for future LoCs.

Figure 1

Figure 1. Schematics of LoC life cycle and examples of environmental impacts.

There is no general consensus regarding the materials for the manufacturing of microfluidic chips, because of the large variety of factors that drive the choice, such as fabrication, integration, function, cost and regulations. This has led to a diversity of conventional materials, each with their own strengths and limitations. Among those, glass LoCs are not very common and are mainly dedicated to specific applications requiring high temperature, high pressure or handling aggressive chemicals (Manz et al., 1992; Zhang and Haswell, 2006). Nowadays, the academic development of LoCs is mainly based on the use of polydimethylsiloxane (PDMS), while commercial systems are mostly made from thermoplastics such as cyclic olefin copolymer (COC), polycarbonate (PC) and polymethyl methacrylate (PMMA). Even if PDMS is biocompatible, transparent down to 300 nm wavelength, and suitable for replicating patterns of a few nm by soft lithography (Becker and Gärtner, 2008; McDonald and Whitesides, 2002), the release of uncrosslinked chains and the absorption of small hydrophobic molecules may be limiting for certain biological applications (Berthier et al., 2012; Wang and Burghardt, 2018). Surface and deep treatments of PDMS have been developed to solve these problems, such as polybrene, a positively charged polymer (Lenhart and Kennedy, 2023). More generally, PDMS still poses challenges for up-scaling from laboratory prototypes to large-scale production and is not the material of choice for manufacturers. In terms of environmental concerns, PDMS production involves hazardous chemicals, including volatile and CMR-classified precursors (Mark et al., 2005). This elastomer is also non-degradable, leading to accumulation in the environment (Laubie, 2012) and potentially in biological systems (Mojsiewicz-Pieńkowska, 2014), hence posing an issue in the case of landfill disposal. When incinerated, it generates significant amounts of CO₂, CO, and toxic compounds such as dioxins and polychlorinated by-products (Ongaro et al., 2022). To avoid exposing the populations to pollutants, some incinerators are equipped with filters; but not all of them.

As the materials of choice currently used for most LoCs on the market, thermoplastics offer a cost-effective solution for high-volume production, with good solvent resistance, over 90% optical transparency (Mela et al., 2005), and compatibility with various micro-patterning techniques (Kolew et al., 2011; Hulme et al., 2002; Khan Chantal, 2006; Khorsandi et al., 2021). Their worldwide production weighted 316 Mt in 2023, whereas the recycled or bio-based plastics represented only 39.7 Mt (Plastics Europe, 2024). Bonding to seal the devices has remained challenging for several years, and various direct and indirect methods have been developed. For each thermoplastic, the best suited method must be defined in order to achieve a strong and reliable bond (Giri and Tsao, 2022; Shakeri et al., 2022). Regarding the environmental impacts, these thermoplastics are derived from hazardous precursors whose synthesis requires extreme pressure, temperature, or catalysts (Bauer, 2000; Serini, 2000; Scheirs and Priddy, 2003; Shin et al., 2005). Although the plastic end-products of the LoCs are not directly toxic, their persistence over centuries raises an issue for their disposal in landfills. Additionally, their incineration is just as polluting as in the case of PDMS. Another disposal method could be considered for these polymers: their recycling via a mechanical process (Wan et al., 2017; Popescu et al., 2009) or a chemical process (Ongaro et al., 2018a). While recycling would not eliminate the dependence on non-renewable raw materials, it would reduce their consumption, and it would strongly reduce the amount of waste. As already mentioned above, however, recycling could also necessitate a biological decontamination or a chemical cleaning step depending on the previous applications, which means that its implementations would require a complex waste management (Ongaro et al., 2022). For this reason, although it is an interesting and complementary avenue to this review, LoC recycling will not be addressed here, and this review will be focused on the potential technological solutions for single-use LoCs that would fit in with the current regulation framework.

As stated above, the production and disposal of LoCs made of PDMS and thermoplastics have a major impact on the environment and public health. Reducing the environmental impact of LoCs has been mentioned in a few articles over the recent years, but only a few reviews have pondered the available solutions. Among them, Ongaro et al. (Ongaro et al., 2022) described medical waste management in different parts of the world and the resulting pollution potentially generated by single-use LoCs. They gave a brief overview of more sustainable and less harmful alternatives, based on plastic recycling or the use of bio-based materials. In her analysis of the LCA of LoCs in 2023, G. Core (Core, 2023) highlighted the need for sustainable consumables including the materials for the fabrication of LoCs, as well as the manufacturing processes and chemicals involved. She also questioned the environmental benefits of digitizing LoC data and proposed to intensify greener practices in laboratories. The need for more sustainable LoC materials has also been addressed by Ma et al. (Ma and Xu, 2023) in their 2023 review, which first provided a succinct insight into some bio-based substrate materials for microfluidics, before focusing on a specific application for low-carbon chemical engineering. All these works highlight the complexity and diversity of factors that need to be considered, in order to reach sustainability of future LoC devices. In 2024, an interdisciplinary action call has been published to draw attention to the various facets of this problem, from the materials and reagents used, through economic issues, to the regulations governing manufacture, use and recycling of medical devices (Deman et al., 2024). Among the several actions that need to be taken in parallel, replacing crude oil-based materials with bio-sourced polymers, which are also biocompatible and more biodegradable, appears essential.

In this context, this review aims at providing a more extensive survey of the alternative bio-sourced material solutions for microfluidic systems that are proposed in the literature. For each bio-sourced biomaterial, this review presents the devices that have been proposed, the possible fabrication methods, including patterning and bonding, and the material properties focused on microfluidic systems (transparency, watertightness, biocompatibility …). Moreover, the associated environmental impacts are assessed, both in terms of production and disposal (incineration and biodegradability). It must be pointed out that the impacts of disposal are restricted to the raw materials, without considering the packaging, functionalization, or any of the chemical and technological steps used to transform the raw material into a complete microsystem. To our knowledge, there are no studies on the degradation of complete LoCs made of bio-sourced polymers so far. In the following, the paper-based LoC technology is first presented, with some examples of devices already on the market. Then, emerging bio-based polymers proposed to produce LoCs, sorted into proteins (zein, silk and gelatin), polyesters (shellac and polylactic acid), lignocellulosic materials (wood) and polysaccharides (alginate and chitosan) are described.

2 Commercialized paper-based technology

2.1 From paper-based tests to microfluidic paper-based analytical devices

Paper-based tests have been marketed for decades, with the famous pH paper, and then lateral flow assay (LFA), for example, for pregnancy tests or, more recently, antigenic tests for Covid-19 (Carrell et al., 2019). Paper is made from plant fibers, mainly composed of cellulose. Unlike commonly used papers, which contain additives that are kept secret, microfluidics on paper uses chromatography paper or filter papers whose exact composition is known, with a price tag of around 1–3€/kg (Kumar et al., 2024).

As paper is hydrophilic, fluids are transported passively by capillary force and do not require external injection equipment such as pumps or syringe pumps. Chemical modification of the cellulose in the paper is often required to immobilize probes, such as proteins or nucleic acids for analyte capture (Yamada et al., 2015), and to enable a colorimetric or fluorescent readout (Nnachi et al., 2022). The various markers or reagents can be deposited using a conventional printer, allowing to control the spatial resolution and the quantity deposited. At the end of the assay time, the reading is qualitative and/or semi-quantitative.

The first microfluidic paper-based analytical devices (μPADs) appeared in 2007, with the main evolution from LFA being the implementation of flow channels on the paper, allowing complex sample preparation steps as well as multiplexed detection to be performed on the same device (Whitesides, 2006; Carrell et al., 2019). To spatially control and direct the flow of solutions, hydrophilic zones are separated by hydrophobic barriers. A 3D flow control has been demonstrated by folding the paper and transferring solutions between panels (Figure 2A). The most common method to produce microchannels is wax printing followed by heat treatment to ensure diffusion of the wax through the thickness of the paper (Yamada et al., 2017). Other methods such as photolithography, laser cutting or inkjet engraving are also possible (Kumar et al., 2024). Channels range from a few hundred micrometers to a few millimeters wide (Lausecker et al., 2016). More details can be found in the review of Nishat et al. on paper-based microfluidics (Nishat et al., 2021).

Figure 2

Figure 2. Paper microfluidic and μPADs – (A) Paper-Origami DNA microsystem proposed for the detection of 3 targets (spots 1, 2, and 3) with internal positive (P) and negative control (N). The device consists of 5 panels folding onto each other and made from filter paper with wax-printed fluidics channels (I) mounted on a plastic plate (II). The sample is introduced into the glass fiber (III) and the analytes are extracted and then eluted towards the first panel containing the detection chambers by folding the paper. The signal is read out under UV flashlight after amplification. Reproduced from ref (Yang et al., 2018). with permission from the American Chemical Society, copyright 2018; (B) Natural indigenous paper substrates for colorimetric bioassays. Reproduced from ref (Brito-Pereira et al., 2023). with permission from the American Chemical Society, copyright 2023.

The systems’ low cost and ease of use by untrained operators make them attractive to the general public. µPADs are increasingly deployed for complex analytical procedures, combining the pre-treatment of whole blood or food samples with easy visualization of results (Fu et al., 2023; Kitchawengkul et al., 2021; Behera et al., 2025; Arslan and Trabzon, 2025). However, they present several limitations. Although µPADs are the most advanced in terms of industrialization, they cannot cover all applications, including cell manipulation and detection of very low analyte concentration. This is because paper microfluidics offers limited control over fluid flow and has poor chemical and solvent compatibility. Regarding the colored signals, the interpretation of the results may depend on the observer and ambient conditions such as light (Hong and Chang, 2014). In addition, the effect of environmental conditions on these open systems, such as temperature and ambient humidity, can lead to variability impacting the reproducibility of results. To improve sensitivity, an external detector such as a camera can be added. In this respect, the integration of smartphone technologies for data reading and analysis could make these devices even more attractive to end-users (Zhang et al., 2021; Xiong et al., 2022; Yüfer et al., 2022). The size of the paper-based microfluidics market, estimated at around $1.2 billion in 2023, is expected to reach around $4.8 billion by 2032 (Sharma, 2025). Target applications include not only the health sector, but also environmental monitoring and food safety (Figueredo et al., 2020).

2.2 Environmental discussion and alternatives

In the light of the above estimates for future production volumes, the environmental impact of producing µPADs should be examined more carefully. Paper production is very water-intensive, especially during pulp production. On average, it takes 500 m³ of water to produce one ton of paper (Olejnik, 2011). The use of chlorine-based bleaching products contributes to water, air and soil pollution. Recycling paper reduces the consumption of these raw materials, but 10% of paper is lost in each recycling cycle. Mandeep et al. have highlighted some recent techniques for reducing pollutants from the pulp and paper industries, such as microbial fuel cells, anaerobic digestion and bleaching technologies (Mandeep et al., 2019). Research has also recently focused on developing new microfluidic substrates to replace paper for ecological purposes. R. Brito-Periera et al. have developed microfluidic substrates using cellulose extracted from abaca, kozo, linen, cotton and sisal with “greener” processes (Figure 2B) (Brito-Pereira et al., 2023). These various plants grow on different continents and offer the possibility of a local supply of cellulose. Biodegradable polymers (Mettakoonpitak et al., 2021; Brito-Pereira et al., 2022a) and other materials such as silk (Brito-Pereira et al., 2022b), hemp (Temirel et al., 2021) or bamboo (Kuan et al., 2016) have also been considered as substrates for µPADs.

Paper itself is biodegradable and flammable, so it can be disposed of by incineration or landfill with no significant toxic emissions. However, paper microfluidic devices are often encased in plastic, the impact of which has been discussed previously in this review, both in terms of manufacture and disposal. Studies are underway to develop packaging made from bio-sourced or recycled polymer, such as the biodegradable housing for lateral flow assays designed by the company Okos Diagnostic® (Anon, 2012).

Although they appeared shortly after the development of µPADs, thread-based analytical devices (µTADs) remain fairly unknown by comparison but are worth mentioning (Tan et al., 2021). Textile threads, unlike paper, are naturally in the form of channels and therefore do not need the creation of barriers to transport the liquid. They also present better mechanical properties than paper in both dry and wet environments, and they are light and flexible, allowing networks to be easily constructed with knots and tangles to obtain microfluidic circuits (Agustini et al., 2021). Threads can be obtained through synthetic or semi-synthetic fibers such as nylon, acrylic, polyester, rayon, or natural fibers such as cotton, wool or silk (Agustini et al., 2021). An eco-responsible way to obtain those threads could be the recycling of used textiles.

3 Emerging bio-sourced materials for microfluidics

In the recent years, bio-based polymers have emerged as promising candidates for the manufacture of complex LoCs. In addition to being biodegradable and intrinsically biocompatible, bio-based polymers can be shaped without organic solvents and under near-ambient temperature and pressure conditions. This section presents the different biopolymers that have been investigated to fabricate LoCs, sorted into protein-based (zein, silk and gelatin), polyester-based (shellac and polylactic acid), lignocellulose-based (wood) and polysaccharide-based (alginate and chitosan). Note that, due to their early stage of development, the properties of these LoCs are bound to evolve with future research. Similarly, there is a lack of data on the biodegradability and/or incineration of these new LoC systems, which means that only the environmental impact of bio-sourced materials will be discussed here, and not the impact of the LoC as a whole.

3.1 Proteins as raw materials

Proteins are complex macromolecules performing a vast array of functions in living organisms. They are highly organized structures composed of folded amino acid sequences. Some proteins can self-assemble into ordered quaternary structures, which in turn form fibrous and robust materials. Their natural origin and structural versatility have made them attractive building blocks for developing bio-based materials that can integrate seamlessly with biological systems. Moreover, proteins are natural biodegradable materials, whose degradation products, i.e., amino acids, can be bioassimilated. The main characteristics of the fibrous proteins, such as zein, silk and gelatin, used for the fabrication of LoCs are described below.

3.1.1 Zein

A prolamine protein found in corn, zein can be extracted as a by-product of ethanol production from corn and processed into resin. This protein forms a hydrophobic thermoplastic with a glass transition temperature (T_g) between 139 °C and 165 °C. Zein is non-toxic, biocompatible and biodegradable. The biocompatibility of zein film was studied with human liver cell and mouse fibroblast cell cultures by Dong et al. (Dong et al., 2004). Formulation as a food packaging to delay food degradation is one of the most developed applications (Corradini et al., 2014). The first zein LoCs were obtained by Luecha et al. in 2011 (Luecha et al., 2011) (Figure 3), but, as far as we know, no other study for zein LoCs has been published.

Figure 3

Figure 3. Zein-glass and zein-zein microfluidic devices and a colorant-filled zein-glass microfluidic device with tubings. Reproduced from ref (Luecha et al., 2011). with permission from the Royal Society of Chemistry, copyright 2011.

Insoluble in water, zein is dissolved in aqueous ethanol, isopropanol or acetone before being cast in a mold. Patterns can be transferred by soft lithography. Initial results, obtained by Altunakar et al. in 2010, showed a resolution of pattern transfer down to 30 µm (Altunakar et al., 2010). In the case of the microsystem demonstrated by Luecha et al., the sealing was obtained via bonding of the patterned zein film to another zein film or to a glass slide, using a process based on partial dissolution of the surface in contact with a solvent (Luecha et al., 2011). A zein film can also be produced by hot pressing at 140 °C (Liu et al., 2010), but the high temperature can induce protein denaturation. This process has not yet been applied to micro-patterning, to the authors’ knowledge.

It is possible to selectively increase the hydrophilicity of the surface of a zein film, by changing the surface properties of the molds, for instance by using a PDMS mold pre-treated with oxygen plasma (Gezer et al., 2015). In addition, the water absorption of these films depends on the type and concentration of plasticizers used, such as glycerol, which increases the water absorption of zein films (Lawton, 2004).

One drawback of zein is its opacity when in contact with water. In 2014, Han et al. studied the optical properties of 100 µm thick zein films and showed that the transmittance of the film was falling below 1% upon hydration, whereas the dry film had a transmittance of over 91% between 450 and 800 nm (Han et al., 2014). In order to maintain the transparency of hydrated films over 87% and reduce film swelling, they proposed a 20 min treatment at 121 °C, 100% relative humidity and 103.4 kPa (Han et al., 2014). This treatment has not yet been applied to LoC manufacture.

Zein is potentially available in large amounts as by-product of biofuel processing activities, with a current price tag of around 20–50€/kg (Shukla and Cheryan, 2001). However, a LCA study by Jones et al., which has been conducted in the case of zein films for packaging, estimated that the production of zein via the above process yields a higher environmental impact than any of the subsequent steps in the life of the zein films (Jones et al., 2020). This means that the processes involved in zein production remain to be improved. To the authors’ knowledge, there is no available information on the environmental impacts of the incineration of zein films yet. In the case of outside disposal, biodegradation of zein takes place within a few months, thanks to the micro-organisms present in the soil (Spence et al., 1995). In vivo, zein degrades in 2 weeks (Bayer, 2021).

3.1.2 Silk

Silk is a natural protein fiber composed mainly of fibroin, the structural center of the silk and sericin, the gum coating the fibers and allowing them to stick to each other. Silk is industrially produced from Bombyx mori cocoons, which are first boiled to remove the sericin gum. Degumming treatments can involve acidic, basic, soapy or enzymatic solutions (Chen et al., 2019). Cocoons are dissolved in a solution of lithium bromide (LiBr). Dialysis is then required to isolate the silk from inorganic ions such as Li⁺ and Br⁻. Finally, a centrifugation step retains the undissolved silk particles. The price of medical-grade silk (for tissue engineering, drug delivery systems, surgical sutures …) is estimated at between 100€ and 300€ per kg.

By mixing the aqueous silk solution with horseradish peroxidase and H₂O₂, silk can be cross-linked to form a hydrogel (Partlow et al., 2014). Such a hydrogel can be micro-structured using soft lithography. After patterning their silk film using a PDMS mold, Bettinger et al. successfully demonstrated bonding of the micro-patterned hydrogel to another silk hydrogel to produce a microsystem. Their proposed bonding process used a mechanical pressure at 70 °C for 18h with aqueous silk solution between the hydrogels (Bettinger et al., 2007). To circumvent the issue of bonding, Zhao et al. (Zhao et al., 2016) used a gelatin mold in the shape of the desired microchannels and cross-linked silk all around it. By injecting hot water, the mold was subsequently melted to free the channels and produce a functional microfluidic system, with a resolution of 100 µm. They encapsulated their system in a PDMS packaging with an acrylic cover containing the tubing adaptors, to provide a mechanically sturdy case around the soft silk hydrogel (Zhao et al., 2016) (Figure 4A). Alternatively, Applegate et al. showed that patterns could also be generated inside the hydrogel with an ultrafast laser pulse without crosslinking (Applegate et al., 2015) (Figure 4B). This fabrication method bypasses the bonding step to seal the microsystems.

Figure 4

Figure 4. (A) Silk microfluidic device in PDMS packaging with gradient generators on 2 different layers (red and green; blue and yellow). The stacking of 2 chemical gradients in the vertical direction creates a 2D concentration gradient map of the 4 colors (Scale bar, 1 cm) Reproduced from ref (Zhao et al., 2016). with permission from Elsevier, copyright 2016; (B) Silk hydrogel patterned by laser ablation and used for 3D culture of human foreskin fibroblasts during 5 (i) and 8 days (ii) The arrow indicates cells growing along the micro-patterned lines (Scale bar, 100 µm) Reproduced from ref (Applegate et al., 2015). with permission from PNAS, copyright 2015; (C) Microfluidic channel in silk hydrogel etched with water and turned into gel by freeze-drying before (i) and during peeling off of the gel layer in the channel (ii). Morphology of the freeze-drying gel layer (iii) Reproduced from ref (Zhou et al., 2022). with permission from Elsevier, copyright 2022.

More recently, Zhou et al. proposed another fabrication method that consists in casting the silk solution in a mold to obtain a water-soluble silk film. A subsequent 40 min treatment with ethanol makes the film water-insoluble by modifying the structural organization of the polymer chains (Zhou et al., 2022). They showed that it is possible to, etch the silk film with either LiBr solution after the treatment (Zhou et al., 2021) or with water before the treatment (Zhou et al., 2022) to engrave concave microfluidic channels of 100 µm of diameter (Figure 4C). The sealing of the device was achieved using silk solution as a glue.

Silk hydrogel offers adjustable mechanical properties (Young’s modulus from 1 kPa to 1 MPa) depending on the concentration of the cross-linking enzyme, horseradish peroxidase (Zhao et al., 2016). The silk hydrogel exhibits optical transparency of over 90% between 400 and 700 nm, excellent biocompatibility and a predictable degradation rate depending on the diameter of the silk fibers (from a few hours to several years). Biocompatibility was tested by culturing human umbilical vein endothelial cells in porous silk microtubes obtained from a mixture of silk fibroin solution and poly (ethylene oxide), which is then dissolved in hot water to create pores (Lovett et al., 2008).

In their study, Zhao et al. (2016) bio-functionalized the silk hydrogel by loading it with alkaline phosphatase (ALP) during the manufacturing process. They demonstrated a channel-bulk diffusion in the system as, 30 min after the injection, para-nitrophenyl phosphate (a soluble reagent for ALP) diffused through the hydrogel and reached another channel 1 mm away (Zhao et al., 2016). This phenomenon of diffusion in silk hydrogel was used for 3D cell culture with a 3D perfusion network (Zhao et al., 2016). Microfluidic channels would enable the perfusion of the cell culture medium that provides nutrition for the cells.

Apart from the ethical issue of silk production, the use of reagents such as LiBr (corrosive, psychoactive effects) and H₂O₂ (oxidizing, toxic, corrosive) has a strong negative impact on the environment and human health. In order to limit the environmental impact of silk production, Lu et al. have suggested recycling methods for silk textiles or silk production wastes, enabling applications in biological tissue engineering, filtration of air and water, or electrodes (Lu et al., 2022). As shown by the same authors, silk incineration produces 10 tons of CO₂ per ton of textiles burned (Lu et al., 2022), but no relevant information on potential other toxic emissions has been found in the framework of this review. In nature, silk is degraded by soil bacteria like P. cepacia within a few months (Seves et al., 1998).

3.1.3 Gelatin

Gelatin is obtained from the hydrolysis of collagen, a fibrous structural protein found most abundantly in the skin, smooth connective tissues and bones of animals. Thus, gelatin is industrially extracted as a side-product in the food industry. Besides the original sources–mostly porcine and bovine–several alternative gelatin sources have recently been on the forefront, like fish and poultry gelatin (Talha et al., 2024). The raw material goes through a degreasing step with hot water and a pre-treatment in an acidic or alkali solution followed by the extraction of the protein with boiling water, the evaporation of the liquid to separate it from the solid gelatin that is then grinded into powder (Alipal et al., 2021). The chemical action of the pre-treatment and the heat during the extraction enable the conversion from collagen to water-soluble gelatin by breaking non-covalent and covalent bonding in the protein structure. According to the tissue type and the species of origin, as well as the pre-treatment applied, gelatin presents different functional groups and molecular weight (Alipal et al., 2021). Gelatin is widely used in cell culture due to the remaining biological cues from collagen promoting cell adhesion (Song et al., 2022). Therefore, multiples studies have proven the biocompatibility of gelatin with animals and human cells (Catano et al., 2025; Sasaki et al., 2023; Salehi et al., 2024; Song et al., 2022; Paguirigan and Beebe, 2006), as well as bacteria like E. Coli (Treebupachatsakul et al., 2022). The price of medical-grade gelatin (for tissue engineering, drug delivery, implants …) is between 5 and 9 €/kg.

Gelatin proteins can form a physical hydrogel with hydrogen bonding at ambient temperature. Even if its melting temperature depends on the grade and concentration of gelatin, these gelatin gels do not usually hold their structure above 30 °C (Golden and Tien, 2007). For applications that do not involve a controlled dissolution of the gelatin, gelatin molecules are crosslinked, using commonly crosslinkers such as (1) microbial transglutaminase (mTG), an enzyme catalyzing trans-amidation between glycin and lysin residues on gelatin (Sasaki et al., 2023), (2) genipin, a plant-derived aldehyde compound (Makita et al., 2018), or (3) methacrylic acid with radical reactions (Catano et al., 2025). Compared to the two other crosslinkers, methacrylic acid is not of biological origin and its uncrosslinked molecules present severe cytotoxicity (Niu et al., 2021).

Micropatterns are obtained on gelatin hydrogel with either soft lithography (Štumberger and Vihar, 2018; Sasaki et al., 2023; Salehi et al., 2024; Paguirigan and Beebe, 2006; Treebupachatsakul et al., 2022) or 3D printing (Catano et al., 2025; Song et al., 2022). For instance, for soft lithography, Sasaki et al. have produced a channel of 200 µm wide and 300 µm height with a pattern surface of 20 µm resolution with a mTG-crosslinked gelatin hydrogel on a PDMS mold (Sasaki et al., 2023) (Figure 5). Patterns have been obtained with a resolution as low as 500 nm for a genipin-crosslinked hydrogel (Makita et al., 2018). In order to seal these microfluidic devices to gelatin, glass or PDMS, uncured gelatin solution with the crosslinker can be used as glue (Makita et al., 2018; Treebupachatsakul et al., 2022; Paguirigan and Beebe, 2006). As for 3D printing, Catano et al. have developed a scale-down model of patient-specific carotid artery in gelatin methacrylate with a channel of 1 mm of diameter (Catano et al., 2025).

Figure 5

Figure 5. Gelatin microfluidic device with inlet tubes produced by soft lithography on a PDMS mold. Reproduced from ref (Sasaki et al., 2023). with permission from MDPI, copyright 2023.

Crosslinked gelatin is optically transparent, expect in the case of genipin. Treebupachatsakul et al. have designed a fluorogenic test for detecting E. Coli via their β-D-glucuronidase (GUD) activity (Treebupachatsakul et al., 2022). The absorption of UV-light below 400 nm was used as a filter to remove the excitation peak of a resulting fluorescent compound from the emission peak. Moreover, the autofluorescence of collagen (excitation peak at 320 nm, with an emission peak at 400 nm) is partially retained by gelatin but should not interfere with common wavelengths of interest in fluorescent microscopy (Paguirigan and Beebe, 2006). To obtain a fluorescent signal, Treebupachatsakul et al. leaned on the diffusion of the GUD enzyme through the gelatin hydrogel loaded with specific target (Treebupachatsakul et al., 2022). Indeed, gelatin hydrogels are permeable to water and other molecules enabling, among other things, the replenishment of nutrients from culture medium. Once immerged in aqueous solution, it is also subject to swelling, with an increase of 15%–20% in external dimensions and 3.5% for the channel diameter (Catano et al., 2025).

Gelatin extraction leads to the valorization of waste resulting from the food industry (Pinto et al., 2022). To improve on the conventional extraction methods that use harsh chemical treatment and cause quality degradation, several greener approaches are studied like hydrolysis using proteolytic enzymes instead of acid and alkaline treatments or ultra-sound assisted extraction (Noor et al., 2021; Rigueto et al., 2022; Abedinia et al., 2020).

Gelatin has a rapid degradation process with measurements of a 68% weight after 15 days in soil (de Campo et al., 2017) and a full biodegradation in 50–70 days at 20 °C in marine and soil environments (Mroczkowska et al., 2021). These studies also highlighted the absence of negative long-term impacts, as the degradation products may be metabolized by microbes. In vivo, gelatin can be degraded by enzymes such as collagenase with no toxic degradation products (Kirchmajer et al., 2013; Tondera et al., 2016). No major negative impact of its incineration has been reported to the authors’ knowledge.

3.2 Polyesters as raw materials

Polyesters constitute a versatile family of polymers characterized by ester bonds in their main chain, which provide a unique combination of mechanical strength, thermal stability and tunable degradability. Their versatile properties and ease of processing have made them widely used in applications ranging from textiles and packaging to biomedical materials. Traditionally, most polyesters are derived from fossil-based monomers. However, increasing environmental concerns have driven the development of bio-sourced polyesters from renewable feedstocks such as plant-derived sugars, fatty acids or microbial metabolites. These bio-based materials preserve the performance of conventional polyesters while offering a reduced environmental footprint. Owning to these advantages, bio-based polyesters, like shellac and polylactic acid, have been used to produce LoCs.

3.2.1 Shellac

During its life cycle, the Kerria Lacca insect, a species of cochineal, secretes a resin called lacquer. Refining this lacquer by heating yields shellac, a thermoplastic with a T_g of 42 °C. Structurally, shellac is a low-molecular-weight resin mainly composed of oxyacid polyesters. The oxyacids are divided into aleuritic acids and cyclic terpene acids (jalaric acid, shellolic acid) linked by ester bonds, which, respectively, constitute the hydrophobic and hydrophilic components of shellac (Yuan et al., 2021). Its dark brown color can be suppressed by bleaching with mainly NaClO or H₂O₂ (Yuan et al., 2021).

Shellac is naturally insoluble in water, but soluble in certain alkaline solutions and organic solvents. The most widely used solvent is ethanol, enabling the production of thin films (from 10 nm to 10 µm thick), often used as a protective layer for active substances (Phan The et al., 2008).

Shellac LoCs were first developed by Lausecker et al. in 2016 (Lausecker et al., 2016), but it seems that no subsequent studies have reported further developments in this area. They first deposited shellac films on paper to protect them during the hot-embossing process (Lausecker et al., 2016). Next, a mold was pressed at 50 °C and 4 kN to print patterns ranging from 30 to 100 µm wide. The system was finalized by heat-treating another shellac film at 58 °C for 10 min. An example of the resulting microfluidic system is shown in Figure 6A. The shellac films are hydrophilic and transparent, but the presence of the paper backing used in this work reduces the system’s transparency. Another potential fabrication method currently under investigation is 3D printing, as some works reported the possibility of producing filaments from shellac (Chansatidkosol et al., 2022; Chansatidkosol et al., 2023) (Figure 6B).

Figure 6

Figure 6. (A) Shellac microfluidic devices produced by hot embossing shellac/paper bilayer with transparent PDMS stamp. Reproduced from ref (Lausecker et al., 2016). with permission from AIP Publishing, copyright 2016; (B) Shellac printed filaments designed for 3D printing without polyethylene glycol (PEG), prepared using hot melt processes. Reproduced from ref (Chansatidkosol et al., 2023). with permission from John Wiley and Sons, copyright 2023.

Shellac is a biocompatible, non-toxic material. It is also used in many applications as a coating to limit or even prevent mammalian cell adhesion and proliferation (Yang et al., 2022). It can be used as a culture surface by grafting cell adhesion-promoting groups onto the shellac carboxylic acid groups (Sunakawa et al., 2023).

The production of shellac is limited by the insect population and their life cycle, as well as the trees that arbor them (Yuan et al., 2021). The price of medical-grade shellac (for pharmaceutical coatings, control drug release, dental materials …) averages 3–50 €/kg. No relevant information on the environmental impact of its incineration, in terms of CO₂ emission, emission of toxic compounds or pollutants, has been found in the framework of this review. In soil, shellac degrades naturally and slowly, with a loss of mass of around 10% per month (Ghoshal et al., 2009), without negative effect on plant growth (Poddar et al., 2024).

3.2.2 Polylactic acid (PLA)

Lactic acid can be extracted from the fermentation of sugars from corn or sugar cane by bacteria. It is then transformed into high-molecular-weight PLA via lactide ring-opening polymerization catalyzed by tin octoate. PLA is a thermoplastic with a T_g of 60 °C–65 °C.

Several studies have demonstrated PLA’s biocompatibility with endothelial cell culture (Huang et al., 2013) as well as with various cell lines (human hepatoblastoma C3A, breast cancer A549) (Ongaro et al., 2020). Furthermore, Ongaro et al. have shown that PLA does not absorb small molecules (∼300 Da) as opposed to PDMS, and that its transparency is comparable to that of standard materials, with a transmittance of 92% (Ongaro et al., 2020). PLA’s auto-fluorescence is slightly higher than conventional materials but is low enough to not interfere with fluorescence imaging.

PLA is compatible with multiple manufacturing methods such as 3D printing (Tothill et al., 2017), injection molding, hot-embossing and laser ablation (Ongaro et al., 2018c). The variety of micro-patterning methods enables both rapid prototyping using laser ablation or 3D printing, and industrial-scale production, e.g., using injection molding or hot-embossing. In case of laser ablation, a sacrificial layer of tape is used to avoid damaging the substrate surface and is removed afterwards, allowing for the creation of a 1 mm-wide channel (Ongaro et al., 2018c) (Figure 7A). This technique was combined with the integration of graphene electrodes into a PLA microsystem (Ongaro, et al., 2018b), in order to demonstrate biosensing applications (Figure 7B). In case of 3D printing, the creation of a 100 µm cavity has been reported, as well as the production of 3D microfluidic systems with tubing connectors (Duong and Chen, 2018). For instance, a 3D printed system has been combined with the LAMP (Loop-mediated isothermal amplification) method to detect S. pyogenes with results close to laboratory equipment (Uysal et al., 2024) (Figure 7C). To our knowledge, the pattern dimensions obtained with the other methods is not indicated.

Figure 7

Figure 7. (A) PLA microfluidic system micro-patterned by CO₂ laser through a protective layer (removed after engraving), bounded to a thin PLA layer using a welding method based on laser absorption. Reproduced from ref (Ongaro et al., 2018b). with permission from the authors; (B) PLA microfluidic device with printed electrodes from water-based graphene inks (Scale bar, 0.2 mm) Reproduced from ref (Ongaro et al., 2018c). with permission from the American Chemical Society, copyright 2018; (C) 3D Printed PLA LoC used to perform the LAMP method to detect Streptococcus pyogenes. Reproduced from ref (Uysal et al., 2024). with permission from MDPI, copyright 2024.

Various methods are described in the literature for sealing PLA LoCs such as thermal bonding (Ongaro et al., 2018b), chemical activation under UV exposure (Ongaro et al., 2020), and functionalization via CO₂ laser absorption (Ongaro et al., 2018c). A functional system bonded with the latter is shown in Figure 7A.

The average price of PLA is about 5–15€/kg. Concerning its environmental impact, its production involves tin-based catalysts, categorized as toxic, CMR and ecotoxic. We also need to consider the fact that lactic acid production competes with food production. Furthermore, even though PLA is a bio-sourced polymer, it is not degraded after a year in seawater and freshwater at 25 °C (Bagheri et al., 2017). It takes several centuries to degrade under natural conditions. In terms of CO₂ emission, it has been shown that PLA incineration is worse than its landfill disposal (Choi et al., 2018). It is possible to compost PLA in several months in industrial facilities capable of exceeding 60 °C with controlled humidity (Kalita et al., 2019). However, composting PLA emits methane, a powerful greenhouse gas. So, PLA’s environmental impact, while being lower than that of petroleum-based polymers, remains significant.

3.3 Lignocellulose and polysaccharides as raw materials

Polysaccharides are composed of repeating monosaccharide units linked by glycosidic bonds, forming on the most abundant classes of natural polymers. Polysaccharides are naturally widely distributed in nature from plants to microbes through animals. In living organisms, they serve mainly structural support or energy storage functions, contributing to the integrity and organization of cell walls, tissues, and extracellular matrices. Their repetitive molecular architecture enables the formation of stable, often crystalline, networks through hydrogen bonding and self-association, giving rise to materials with tunable mechanical and physicochemical properties. Polysaccharides are inherently biodegradable. These characteristics have made them particularly attractive for designing bio-based materials for biomedical, pharmaceutical, and environmental applications. This section presents LoC applications from wood (a lignocellulosic material), alginate and chitosan.

3.3.1 Wood

Wood is a lignocellulosic material valued for its mechanical, thermal insulation and heating properties in many industrial sectors. It mainly consists of two polysaccharides, i.e., cellulose and hemicellulose, and lignin, an aromatic polymer. According to softwood or hardwood species, wood contains in mass percentage 33%–51% cellulose, 17%–40% hemicellulose and 21%–32% lignin (Tarasov et al., 2018). Many parameters influence its properties, such as the species of origin and the moisture content. Biocompatible, biodegradable and versatile, wood can be structured and functionalized to obtain the desired properties, such as water tightness and transparency (Farid et al., 2022). The biocompatibility of wood has been demonstrated by Song et al. on HEK293 cells grown on flexible wood membranes (Song et al., 2017).

Andar et al. used birch plywood to build a microfluidic system (Andar et al., 2019), the first LoC from wood (Figure 8). Plywood is an assembly of layers of wood a few millimeters thick. Channels were engraved by laser ablation with lateral dimension as small as 100 µm for 250 µm depth (Andar et al., 2019). To prevent analytes seeping through the wood, a surface treatment is required. Coatings of PMMA, cellulose acetate and Teflon have been applied by Andar et al. to make the channels hydrophobic and watertight, with better results for Teflon (Andar et al., 2019). However, Teflon is a persistent pollutant that accumulates in the environment, and whose incineration emits greenhouse gases a thousand times more harmful than CO₂. Research should be continued to avoid the use of materials such as PMMA and Teflon. A few more eco-responsible alternatives have been suggested, such as vegetable oils or beeswax (Thakur et al., 2019), but have not been studied yet, to our knowledge.

Figure 8

Figure 8. Microfluidic devices made of wood that have been engraved with CO₂ laser ablation and bounded to a PMMA layer with double-sided adhesive (left, Scale bar, 1 mm) and to another birch plywood with cyanoacrylate glue (right, Scale bar, 3 mm) Reproduced from ref (Andar et al., 2019). with permission from the American Chemical Society, copyright 2019.

The wood can be bonded with medical-grade cyanoacrylate glue to another plywood plate or a glass slide, or with double-sided adhesive to a PMMA plate for fluid visualization (Andar et al., 2019). In such a wooden LoC incorporating a transparent observation window, fluorescent signals were observed in the presence of bacteria. Their detection was improved by eliminating spurious signals generated outside the measurement zone (Andar et al., 2019). However, for other applications requiring full device transparency, wood opacity is a problem. To tackle the issue, Zhu et al. developed transparent wood composites by removing the colored lignin with a soda or H₂O₂ solution and replacing it with an optically transparent polymer like epoxy resin (Zhu et al., 2016). The synergy of these multiple studies on wood could pave the way towards greener alternatives for the development of transparent LoCs based on wood.

Wood has a porous structure of many anisotropic hollow cells oriented in the same direction. Some applications have used this natural microscopic structure as microfluidic channels, for instance, to produce a fluid catalytic reactor from basswood (Shen et al., 2022) or 3D solar vapor-generation device for water desalination from bamboo (Gong et al., 2021). The lignocellulosic walls of the microchannels can be functionalized to immobilize different catalysts or conductive materials (Pandoli et al., 2022).

Wood is a biodegradable material, and also an abundant material, already widely used in industry. It is often used to generate energy from its incineration, costing 2–5 €/kg. Wood waste can be combusted for heat production or power generation (Elginoz et al., 2024), but also releases various air pollutants, resulting in the production of ash, heavy metals, and other pollutants (Oguntoke et al., 2013).

3.3.2 Alginate

Alginate is a linear copolymer with homopolymeric blocks of (1–4)-linked β-D -mannuronic acid (M) and α-L-guluronic acid (G) residues, covalently linked together in different sequences. Conventional alginate extraction from brown algae is usually performed using the pre-treated macroalgae with slightly acidic solutions, followed by alkaline extraction, solid/liquid separation, precipitation, drying, and particle size reduction by milling. Medical-grade alginate (for wound dressing, drug delivery systems, tissue engineering, medical implants …) is estimated to cost between 50 and 100 €/kg.

Negatively charged alginate chains form a hydrogel by forming ionic bonds with bivalent cations. The most used cations are Ca²⁺, but larger metal ions such as Cu²⁺ have been reported to produce more compact hydrogels, thereby decreasing the diffusion of liquids or gases through the hydrogel (Mikula et al., 2019; Drury et al., 2004). Due to ionic cross-linking, the alginate hydrogels become water insoluble.

To fabricate channels, Yajima et al. used soft lithography to gel a solution of sodium alginate (NaA) and propylene glycol alginate (PGA) with a solution of CaCl2 on an SU-8 mold (Yajima et al., 2014). Unlike NaA, PGA is non-gelling and generally used to adjust the physical rigidity of hydrogels. The microfluidic channels obtained were 200 µm wide by 100 µm high (Yajima et al., 2014) (Figure 9A). The hydrogel appears visually transparent, but no transmittance value was given by the authors. Alginate can also be structured by 3D printing by immersing alginate solution filaments in gelling agent like a CaCl₂ solution (Gao et al., 2015).

Figure 9

Figure 9. (A) Ca-alginate hydrogel microfluidic system with tubings injected with an aqueous solution with a blue dye. The system was micro-patterned with soft lithography on SU-8 and silicon wafer and bounded to a PLL-coated Ca-alginate hydrogel plate. Reproduced from ref (Yajima et al., 2014). with permission from AIP Publishing, copyright 2014; (B) Chondrocyte-seeded microfluidic scaffold with fluorescent dye delivery for 2h. The system was made from Ca-alginate hydrogel pre-loaded with cells (visible on the right) and micropatterned by soft lithography with a silicone stamp. The perfusion channels were sealed with aluminium jig. Reproduced from ref (Choi et al., 2007). with permission from Springer Nature, copyright 2007.

The microsystem can be sealed by stacking another hydrogel layer with tweezers to reinforce a mechanical contact (Choi et al., 2007) or by partially dissolving their interfaces (Cabodi et al., 2005). In addition, coating hydrogels with poly-L-lysine (PLL) creates electrostatic bonds between them (Yajima et al., 2014). This last bonding method avoids the potential channel deformations present in the other two methods and withstands injections at a pressure of around 3 kPa. To improve the bonding strength while maintaining a transparency sufficient for observation, Nasiraee et al. combined alginate with agarose to form a hydrogel (Nasiraee et al., 2023).

Using soft lithography at room temperature, Choi et al. produced 100 µm-wide microchannels in alginate seeded with cells and studied the distribution of soluble chemicals through the hydrogel (Choi et al., 2007). They observed that the microfluidic channels allowed for efficient exchange of solutes and for quantitative control of the soluble environment in which cells evolve in their 3D environment (Figure 9B). Nasiraee et al. separated the cultivation channels from the channel with nutriment to avoid any sheer stress on the cells, and used the diffusion between the channels to transfer nutriments to the cells (Nasiraee et al., 2023). As alginate hydrogel generally is not cell adhesive, specific treatments are needed, such as adding PGA (Kwon and Peng, 2002) or a fibronectin coating (Tanaka et al., 2005). However, a loss of metal ions has been observed in hydrogels, resulting in mild cytotoxicity (Mikula et al., 2019; Lee et al., 2013).

Conventional alginate extraction processes require large amounts of acid and alkali, as well as toxic chemicals like formaldehyde to facilitate the pre-treatment (Saji et al., 2022). Enzymatic extraction and pre-treatment methods have been proposed to reduce their environmental impact (Bojorges et al., 2023). Alginate forms a biodegradable hydrogel thanks to its ionic mesh. In vivo, alginate hydrogel undergoes a fall in shear modulus within 2 days, while the surface structure is maintained for 28 days (Shahriari et al., 2016). Alginate is degraded by an enzymatic reaction produced by bacteria living in soil and water, with a loss of over 45% of mass in 40 days (Gacesa, 1992; Phang et al., 2011). The impact of its incineration, in terms of CO₂ production, is evaluated to about 3 kg of CO₂-eq per kg of alginate-based plastic (Ayala et al., 2023). Other impacts of its incineration, such as toxic emissions and pollutants, have not been detailed in the above works.

3.3.3 Chitosan

Chitosan is a polysaccharide derived from chitin, mainly present in the exoskeletons of arthropods and the endoskeletons of cephalopods, as well as in the cell walls of some mushrooms (Kaisler et al., 2020). Chitin can be extracted from already existing waste generated by the seafood industry (Kaisler et al., 2020), which have undergone demineralization step using acidic solutions, deproteinization via alkaline treatment and decolorization to remove pigments and obtain colorless pure chitin. After conversion from chitin to chitosan achieved by another alkaline treatment, the final product is a linear polysaccharide of randomly distributed N-acetyl-d-glucosamine and d-glucosamine units, which are connected by β-(1,4)-glycosidic bonds. Its price (for wound dressing, drug delivery, antimicrobial coatings …) is around 50 to 100 €/kg, depending on the origin of the chitosan (shrimp, crab, squid, etc.), the country of production and the desired quality. As with alginate, solutions are being studied to replace some of the above-mentioned harsh chemical treatments with enzymes or bacteria to reduce the impact on the environment (Younes and Rinaudo, 2015; Chen et al., 2016).

Chitosan is insoluble in water at neutral or basic pH but becomes soluble in dilute aqueous acid solutions. These solutions can be cast and dried to obtain a chitosan film. A neutralization step can be performed to prevent the dissolution of films in contact with water by immersing the film in an alkaline solution (Mattotti et al., 2017; Korniienko et al., 2022) or exposing it to an alkaline gas (Montembault et al., 2005). Zimmer et al. (2024) produced the first microfluidic systems from chitosan films in 2024. They obtained 400 µm thick films and neutralized them for 45 min in 1M NaOH. Despite the neutralization, the chitosan films showed a swelling rate of 49% ± 1% after 30 min of immersion in water, which means that the process still needs to be optimized. The transparency was measured at 75% between 400 and 800 nm for a 400 µm thick chitosan film, but it may vary depending on the origin of the chitosan.

Zimmer et al. have used micro-drilling and hot-embossing to produce microfluidic channels on chitosan films after neutralization (Zimmer et al., 2024). They obtained channels with widths ranging from 105 to 1,000 µm and heights from 50 to 200 µm. To seal the systems, a dry resin was laminated on the chitosan film, which enables its bonding to another chitosan film or a glass slide (Figure 10A). Other patterning methods have produced patterns on thin chitosan films, such as soft lithography (Mattotti et al., 2017), photolithography (Servin et al., 2023; Sysova et al., 2023) (Figure 10B), oxygen plasma dry etching (Cheng et al., 2007) and electrodeposition (Yi et al., 2005).

Figure 10

Figure 10. (A) Chitosan microfluidic system micropatterned with micro-drilling and sealed with a dry resin, before being injected with aqueous solutions colored with food dyes. Reproduced from ref (Zimmer et al., 2024). with permission from MDPI, copyright 2024; (B) Micro-patterns on chitosan film obtained by photolithography. Reproduced from ref (Sysova et al., 2023). with permission from John Wiley and Sons, copyright 2023.

Chitosan is a biocompatible and biodegradable polymer. Numerous studies of in vitro cell cultures in the presence of chitosan (He et al., 2011; Mattotti et al., 2017) and in vivo implantation of chitosan films (Rodríguez-Vázquez et al., 2015) have verified its biocompatibility. In contrast, chitosan has also shown antibacterial and antifungal activities that may be due to interactions between the positively charged groups of chitosan and the negatively charged cell walls of bacteria (Khoushab and Yamabhai, 2010). Chitosan can be degraded in vivo by several enzymes like lysozyme (Szymanska and Winnicka, 2015). The by-products are non-toxic and non-antigenic (Khoushab and Yamabhai, 2010; Rodríguez-Vázquez et al., 2015; Koev et al., 2010). Similarly, soil bacteria produce chitosan-degrading enzymes (Sawaguchi et al., 2015), which completely degrade chitosan in 6 months (Makarios-Laham and Lee, 1995). Finally, some studies show that no toxic by-products are generated during its incineration (Sirviö et al., 2021; Eulalio et al., 2019).

4 Discussion and perspectives

PDMS and thermoplastics, as well as their fabrication methods, have become the standard for LoC production in academia and industry, respectively. Their transparency, impermeability and compatibility with micro-patterning have enabled the development of numerous laboratory applications (Descamps et al., 2022; Faivre et al., 2020; Sun et al., 2019; Zhu et al., 2020; Liu et al., 2019; Lee et al., 2024). However, the extraction of the raw materials, the production of these polymers using harsh chemicals and conditions as well as their disposal by incineration or in nature, contribute to the depletion of natural resources, climate change and the growing pollution of the environment.

Bio-sourced materials represent an attractive alternative to conventional petroleum-based polymers for the manufacture of LoCs. Paper-based microfluidic is already an advanced technology and paper-based products are already on the market. Various other bio-sourced materials have been evaluated for the manufacture of LoCs. Table 1 summarizes the main properties of the different bio-based polymers that have been considered in this review, such as their transparency, watertightness, associated micro-patterning techniques and the dimensions obtained, as well as their environmental impact in terms of production, incineration and biodegradability. All of them are biocompatible. Depending on the materials and their respective mechanical properties, different microfabrication techniques were used to produce the microchannels, the most common being soft lithography, 3D printing, hot-embossing, and laser ablation, but there is no clear tendency indicating that a particular technique would be more adapted to a given family of materials. Similarly, the resolutions of the patterns seem mostly related to the techniques and are in the order of a few tens of µm across all material families, except for genipin-crosslinked gelatin, with which patterns down to 500 nm have been obtained. In terms of transparency, silk, PLA, and chitosan allow monitoring under a microscope in the visible range, although the transparency of the latter is lower than that of thermoplastics or PDMS. Wood and shellac, which is associated with a paper substrate, are not transparent at their current stage of development. It should be noted that most bio-sourced polymers are not leakproof and tend to swell in the presence of aqueous solutions. Only the polyester-based materials, PLA and shellac, are watertight.

Table 1

Table 1. Properties and environmental impacts of bio-sourced materials proposed for the fabrication of LoCs (H-E = hot-embossing, SL = Soft lithography, PL = photolithography).

Comparing the current environmental impacts of these bio-based materials, it is generally highlighted that they can help reduce greenhouse gas emissions, energy consumption and plastic pollution, particularly in the context of single use. However, it is also clear that their relative manufacturing processes play a key role. Although zein, gelatin, and chitosan are by-products of existing industries, the production of devices made from chitosan and gelatin, as well as alginate, PLA, and silk, involves the use of toxic chemicals. These production methods could evolve towards more environmentally responsible processes with advances in greener chemistry. So far, devices made from wood require a PMMA or Teflon-based coating to ensure watertightness, which would also need to be replaced. In terms of disposal of the bio-based devices, most of the currently available data is related to the biodegradability of the raw materials, which usually varies from a few days to a few months; this is much better than conventional plastics, except for PLA that might need a few centuries to completely degrade if conditions are not optimal. However, because of the modification of the material properties induced by technological processing, the biodegradability of the complete LoC devices might be very different from that of the raw materials, and special care on this particular issue should be paid in future material and processes developments, in order to preserve the benefit of bio-based polymers for reducing plastic pollution. Regarding incineration, there are only a few studies that have considered all the environmental impacts induced by the incineration of the bio-based polymers, beside their impact on climate change via the CO₂-eq emissions. Other emissions, such as toxic compounds or pollutants, are usually not discussed, which makes it hard–at this stage of the material developments–to provide a clear evaluation of the environmental impacts of future LoC devices based on these new materials, and to compare them with conventional polymers. Hence, these first evaluations of environmental impacts must be completed with LCA considering all the potential impacts and for the whole LoC fabrication processes, including the additional materials and chemicals that are involved, and not only the raw materials. It should be noted that other bio-based materials, which have not been discussed in this review, are now used in tissue engineering (Rosellini and Cascone, 2023). For example, microfluidic scaffolds have been obtained from the polymer poly (1,3-diamino-2-hydroxypropane-co-polyol sebacate), a biodegradable elastomeric poly (ester amide) (Wang et al., 2010), to make resorbable devices for drug delivery and regenerative medicine. Similarly, other bioplastics derived from biogas or biomass are now being proposed for other areas of application and could become of interest for LoCs in the future. Here again, special attention should be paid to the environmental impact of their production and disposal methods. For all materials, it is also necessary to consider the environmental cost of transportation and to promote local production of these materials.

The diversity of proposed bio-sourced materials is a major advantage, both because it could help decreasing future potential tensions on available supply, and because each material could address different LoC applications depending on its properties. Biocompatible but non-watertight materials such as gelatin, alginate and silk offer liquid diffusion are being studied for 3D cell culture and tissue regeneration. Other materials could become watertight, e.g., via surface treatments, as already proposed for wood, which enabled to demonstrate an application of bacteria detection. Due to their low or zero molecule diffusion, PLA and shellac have also been suggested for biosensing applications. As very few works have tackled the issue of producing LoCs from these materials so far, some properties that might currently appear as drawbacks, such as the non-transparency of wood or shellac devices or the swelling of some bio-based materials in contact with water, could either be improved in the near future with new research or be circumvented by finding an appropriate application. However, before being commercialized, LoCs from bio-sourced materials must tackle some challenges. The first step is the reliable, sustainable and cost-competitive production of raw materials with reproducible properties. The relatively high price of bio-sourced materials compared to the conventional materials could remain an obstacle, if mass production at a greater scale does not sufficiently reduce these costs with respect to crude oil-based plastics. Due to their natural origin, these materials are also more prone to variations from batch to batch. This issue should also diminish with the increase in demand for this type of materials and their large-scale production. Similarly, the micro-patterning techniques that have been proposed so far are mainly restricted to low and medium production volumes. Hence, the future studies will have to consider the possibility of scaling up their production to compete with the thermoplastics. The shelf life of the devices is also a key property that has not been investigated yet. Shelf life of biodegradable polymer chips might be significantly lower than conventional plastic ones and should be considered in the development stage of these bio-sourced technologies. And finally, the societal acceptance and potential rebound effects of the bio-sourced materials will have to be considered before larger-scale production could be envisaged. In particular, materials that depend on specific living animals to produce the raw materials, like shellac, silk, chitosan or gelatin that are based on animal waste from food industries, might rise some concerns. Rebound effects, such as a future competition with food production if the demand for some bio-sourced materials became too high, will also have to be carefully weighted.

As a conclusion, the different studies that have been discussed in this review show the diversity of solutions for replacing synthetic plastics in the production of LoCs. For each one of them, technological obstacles remain to be tackled before meeting suitable specifications for both the LoC and their environmental impacts, and research efforts should be pursued in this direction. Finding the most adequate applications for each bio-based material, well-adapted to the material properties, is a key to lowering the corresponding amount of LoC specifications and the number of technological hurdles that still need optimization. The alternative to petroleum-derived polymers could be a panel of complementary bio-based materials for the manufacture of LoCs, which would–whenever possible–be locally sourced to support the local economy and limit transport, and that would not lead to new ecological imbalances induced by the intensive production of a single bio-based material. In this respect, these complementary solutions would be part of a long-term, more robust approach that would promote a better adaptability to application, economic, and environmental conditions (Grumbach and Hamant, 2020). It is clear that this research should be part of a broader issue, also involving regulators and political decision-makers, health authorities, manufacturers and consumers. Some companies have now committed to sustainable development initiatives, including large corporations such as Mitsubishi Chemical with the bio-based polycarbonate Durabio™, or Arkema with the bio-based polyamide 11 Rilsan®, but also many young companies such as Eranova (AlgX), Renature (Phyli®), and Okos Diagnostics with the development of biodegradable cassettes (made from plant-based materials) for rapid LFA tests. Together, all these initiatives will contribute to advancing the field of microfluidics towards new materials. Regulations concerning authorized materials for medical devices and the disposal of used devices, taking biodegradation and recycling/reuse into account, are also a crucial point that needs to evolve to meet the environmental and health challenges of the 21st century.

Author contributions

MZ: Writing – original draft, Writing – review and editing. EL: Writing – review and editing. ST: Writing – review and editing. CJ: Writing – review and editing. A-LD: Writing – review and editing.

Funding

The authors declare that financial support was received for the research and/or publication of this article. The authors acknowledge financial support from INSERM, for the PURECHIP project (ITMO Cancer-PCSI), from the Cancôropéle CLARA, the Auvergne-Rhône-Alpes Region, as part of the sTC FINDER Proof of Concept program, from IngéLyse and iMUST Labex of University of Lyon.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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The authors declare that no Generative AI was used in the creation of this manuscript.

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