Leather is a by-product of meat and dairy production turned into a durable material by tanning, most often with chromium salts. Its mechanical qualities come from the hide's dense, interwoven network of collagen fibres, which tanning stabilises against heat and microbial attack (Covington, 2009). The same process generates effluent and solid waste that are difficult to treat, and the hazards of poorly regulated tanneries are well documented (Dixit et al., 2015; Kanagaraj et al., 2015). Material flow analysis shows how little of the raw hide ends up in the finished article (Joseph and Nithya, 2009). Concern for animal welfare and for the climate burden of cattle has added to the pressure on brands to find something else.

For decades the alternative was coated fabric: a textile backing carrying a layer of polyvinyl chloride or polyurethane. It is cheap and uniform, but it is made from fossil feedstock and does not biodegrade. The newer proposal is a material that is neither animal nor petrochemical. Some candidates use agricultural residues such as pineapple leaf fibre, apple pomace, grape marc or cactus. Others are grown, either as fungal mycelium or as bacterial cellulose, an approach that designers have explored as a way of working with living matter (Karana et al., 2018).

Press coverage of these materials has run ahead of the technical record. Announcements of collaborations with luxury houses are frequent; peer-reviewed measurements are not. This review asks what the published literature supports regarding performance, and what stands between a promising sample and a material that a factory can order by the roll.

This is a narrative review of the published literature. It draws on peer-reviewed studies in materials science, fungal biotechnology and cleaner production, on standard test methods for leather, and on reference works in leather chemistry. It does not claim to be exhaustive, and no formal screening of records was carried out. Trade names appear only where a published study has examined the product concerned. Commercial claims that could not be traced to a published source have been left out. The term "leather" is used here for tanned animal hide alone, in line with the trade definition (CEN, 2022), and the substitutes are called leather alternatives.

The alternatives fall into three families that differ in how the sheet gets its structure. Table 1 summarises them alongside the two incumbent materials.

Leather is specified by physical tests that have been standardised for decades. Tensile strength and elongation are measured on dumb-bell specimens (ISO, 2020), and resistance to repeated folding is measured on a flexometer that creases the specimen many thousands of times (ISO, 2017). Footwear and upholstery buyers add tear strength, abrasion, colour fastness to rubbing, water vapour permeability and finish adhesion. Any alternative offered for the same uses is, fairly or not, judged against these methods.

The case for these materials is environmental, so the quality of the environmental evidence matters. Fungal biotechnology is widely promoted as a route to a circular economy in which low-value residues become useful products (Meyer et al., 2020), and the textile sector has been urged to adopt lower-impact fibres and materials (UNEP, 2020). Product-level assessments are fewer. Hildebrandt et al. (2021) compared production routes for several bio-based leather substitutes and showed that results depend heavily on the energy used in processing and on the share and type of polymer added. Williams et al. (2022) published a life cycle assessment of one commercial mycelium material and reported a lower carbon footprint than the leather benchmarks they used, while also noting the influence of the electricity supply. That study was produced with the manufacturer, which does not invalidate it but underlines how few independent assessments exist.

Two comparisons deserve caution. The burden attributed to leather depends on how much of the animal's impact is allocated to the hide, a methodological choice on which studies differ. And a footprint per square metre says nothing about service life. A bag that lasts a fraction as long as its leather equivalent must be replaced, and the comparison then changes.

Few of these materials have moved from capsule collections into regular supply. The reviewed literature points to barriers of three kinds, grouped in Figure 1.

The technical barriers have been described above: performance below the incumbent, variability, and dependence on synthetic finishes. They are compounded by the absence of test methods written for these materials. Leather standards assume a collagen structure, and coated-fabric standards assume a textile carrier, so results are hard to compare between laboratories and suppliers.

The economic barriers follow from the biology. Growth takes days to weeks in controlled, clean conditions, and capacity is added tray by tray or vessel by vessel. Hides, by contrast, are an abundant by-product, and polyurethane sheet is made on continuous coating lines. A new material must compete with both on price while still paying for its pilot plant. Brands place small trial orders, which keeps unit cost high and prevents the learning that volume brings. Rogers (2003) described how the spread of an innovation depends on its relative advantage, compatibility with existing practice, complexity, trialability and observability. On compatibility in particular the alternatives are at a disadvantage, since cutting, skiving, stitching and edge finishing in footwear and leather goods factories are tuned to leather.

Market and regulatory barriers complete the picture. Several jurisdictions restrict the word "leather" to animal hide, and the trade terminology standard reflects that position (CEN, 2022). Terms such as "vegan leather" also cover ordinary polyurethane, which blurs the distinction that bio-based producers wish to draw. Sustainability claims that rest on unpublished data invite scepticism from buyers and regulators alike. End-of-life routes are unsettled: a coated, reinforced sheet is a composite that is neither readily compostable nor easily recycled.

Four gaps stand out. The first is independent, standardised testing of current commercial grades, repeated as products change, since published comparisons date quickly. The second is durability in use: wear trials of footwear, bags and garments over realistic periods, with attention to flex cracking, delamination and changes in handle. The third is finishing chemistry. Bio-based coatings and crosslinkers that give water and abrasion resistance without a fossil polymer layer would remove the main inconsistency in the present offer. The fourth is transparent life cycle assessment by parties without a commercial interest, with explicit treatment of service life and disposal.

For designers there is a further question, which is whether imitation of leather is the right goal. Karana et al. (2018) argue that grown materials have qualities of their own, and products designed around those qualities, in applications that do not demand the flex life of a shoe upper, may offer a more realistic first market than direct substitution.

The published evidence supports a measured view. Plant-residue composites are available and usable, but many owe their performance to synthetic polymer. Mycelium sheets are the most promising wholly biological option and have improved quickly, yet they do not at present equal bovine leather in strength and flex endurance without reinforcement or coating. Their environmental advantage is plausible and only thinly documented. Adoption is held back less by any single weakness than by the way technical uncertainty, cost and unclear claims feed one another. Open testing and independent assessment would do more for these materials than further announcements.

To be declared by the author(s) before publication.

To be declared by the author(s) before publication.