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LuxeSci Show Notes: S3E4 - How to Make Leather?

Hi Fancy Folks. Welcome back to another episode of LuxeSci, a podcast to reignite your wonder by exploring the intersection of science and luxury. I’m your host, Dr. Lex, former parasitologist, current global health enthusiast and eternal lover of all things fancy.

This week we’re delving into another natural, and controversial fabric, leather. I am sure that most of us have a leather belt, or shoes or a leather bag. It is a ubiquitous fabric that is prized for its strength, durability and versatility. But where does it come from, what is it exactly, how is it made and what new research is being done on and with leather. Let’s find out together!

Background:

Leather is one of the oldest fabrics.. Long before the practice of tanning was invented, humans were using animal skins as coverings. The challenge was that the skins would dry and crack in the cold and rot in the sun
There is archeological evidence from Hoxne, England that suggests humans as far back as 400,000 years ago were using animal hides for protection
Archeologists theorize that bone awls dating back to 82,000 BC from South Africa were tools used to create holes in leather hides
Evidence of leather tanning can be found in what is now Iraq, dating back to 5000 BC
The Ancient Egyptians were proficient leather tanners, suggesting that the technique had been around for awhile and they were perfecting it
Early leather tanning involved smoke, grease, animal brains and bark extracts
The ancient world then moved to vegetable tanning using tannins from tree bark and that process was labor-intensive
There were some downsides though. Vegetable-tanned leather is not water-stable. If you soak it and then dry it, it shrinks (which is actually advantageous for shoe making). If this type of leather is boiled, it will shrink a lot and party congeal, becoming very rigid and eventually brittle. This makes it good for armor and book binding
In fact, this type of leather is the only type the will survive burial in wet environments. This is important for archeologists because leather materials can provide a lot of information about ancient societies
I know that we’re in the background section but I am going to pause here to share some research because i’m a huge nerd and i love citing some research
One group out of the Teesside University in England and Western University in Canada set out to study how archeological leathers degrade.
They used a technique called Fourier-Transform Infrared Spectroscopy
This technique is already widely used by archeologists to study organic preservation in artefacts (e.g. screening for collagen in bone) and specifically to look at leather to assess damage.
Uses infrared light to scan samples and observe chemical properties in those samples.
The research group used experimentally buried leather to mimic leather found at a viking site in England. The samples were excavated at various time points up to 8 months out from burial and were vegetable-tanned using different tannins and buried in laboratory-based wet soil environments. Modern vegetable-tanned leather samples were used for comparison and archeological samples were excavated from Vindolanda site.
They found that the FITR analysis was not substantially different between the experimentally buried leather and the archeological leather suggesting that the experimental burials could be useful models for further research
And interestingly, they found rapid de-tanning of the leather upon burial and that the de-tanning was not detrimental to the preservation of the leather
The 19th century saw a shift to Chromium tanning, which is still used today
Leather is still a popular commodity. An average of 24 billion square feet are produced annually, with China and India as the largest producers
The global market for leather goods was $444B USD in 2022
North america was the largest market
Genuine leather had the largest market share, although synthetic leather is projected to grow substantially
Footwear was the largest segment at 44%
Cows hides are the most frequent used to make leather. In the US, no cows are killed just to make leather, the hides are taken as byproducts of the food production

Science

So what exactly is leather? Leather is the dermal layer of skin from an animal (usually, cows, goats, pigs, sheep), again, usually as byproducts of the meat industry.
Mammalian skin is made up of 2 primary layers: (fun fact: skin is your largest organ)
Epidermis - thin, outer layer that doesn’t have any blood vessels (nonvascularized). It’s role is protection
Consists of flattened keratinocytes that slowly die off and shed
Keratinocyte - a cell that produces keratin (sulphur-containing, fibrous proteins that are insoluble and resistant to degradation. Form filaments that assemble to provide structural support, hair, nails, horns, claws, hooves, quills, scales, feathers and the epidermal layer of skin)
One cool thing about keratinocytes is that they proliferate in the basal layer and as the move up, they flattened and eventually anucleate.
Coated with lipids and organic salts for antifungal and antibacterial purposes
There are also some immune cells lingering around in the epidermis to ID and destroy pathogens
Basement membrane - i put this in because i like the name. It’s essentially a thin layer of fiber between the epidermis and dermis that regulates traffic between the two - kind of a like a row of toll booths
Dermis - vascularized layer of skin (contains blood vessels) - cushions the body from stress and strain and provides tensile strength and elasticity to the skin
There’s a lot going on in the dermis:
Nerve endings to provide a sense of touch
Hair follicles
Sweat glands
Sebaceous (oil) glands
Lymphatic vessels
Blood vessels
Subcutaneous layer - not technically part of the skin
Below the dermis
Purpose is to attach the skin to underlying bone and muscle and supply it with blood vessels and nerves
Contains 50% body fat (padding and insulation for the body)
Leather making process
OK, that was a lot of anatomy, but what about the leather
Essentially leather is the treated dermal layer of animal skin
There are two layers to the dermis (papillary dermis) and the reticular region (sorry, more anatomy)
In leather making, the papillary dermis is called the grain layer and the reticular region the fiber network layer
The collagen (80-85% of the dermis by dry weight) imparts the grain characteristic to leather, that is the characteristic pattern of the skin (think how cow leather and crocodile leather patterns are very different)
How to make leather - 3 stage process (pretanning, tanning and finishing)
This is from a great mini article from the 1980s, written by a a medical student from Bristol University
Pretanning - removal of the skin from the animal and preserving, softening and preparation for the tanning
The unwanted subcutaneous fat, hair and epidermis are removed by mechanical and chemical methods
The fibrous network in the dermal has to be exposed so that the tanning chemicals can act on it
The use of enzymes in this step is very helpful
Enzymes - proteins that help speed up chemical reactions
Akin to laundry detergent, these enzymes penetrate the skin’s matrix and facilitate the removal of salts, fats, oils and dirt
Tanning - there are lots of types of tanning that lead to different qualities in the leather. I’m not going to go into all of them for this podcast so here is the general idea behind them (from my lovely med student written article):
Increase the number of stable bonds between collagen molecules to strengthen the skin and resist bacterial breakdown and oxidation
Increase the number of crosslinks between natural and added oils to maintain suppleness
Filling the spaces between the collagen fibers to allow for the physical properties necessary to mold and tool the leather
Having ingredients in the process be able to reduce the damage due to atmospheric pollutants, human sweat and inhibit bacterial growth.
The most popular method today uses Chromium sulfate. The chromium cross-link the collagen subunits and the whole process increases the space between the protein chains
Vegetable tanning uses tannins found in bark and leaves. The tannins also bind to the collagen proteins and coat them which leads them to be less water-soluble and more resistant to bacterial attack. Additionally, the process makes the skin more flexible. This type of leather is not super flexible though and tends to be used for luggage, furniture, footwear, belts and other clothing accessories.
Various finishing process to achieve the color, feel, gain, etc that is required for that specific leather. These include shaving to get uniform thickness, dying for color and brushing to polish the grain
Environmental impact
The chemicals used in leather tanning (even vegetable tanning) can increase the chemical oxygen demand and total dissolved solids of water if they are not disposed of responsibly.
This will decrease the amount of dissolved oxygen in the water and make it difficult for oxygen-dependent species to survive
Additionally, some processes can convert the chromium used into carcinogenic hexavalent chromium which can be found in runoff and scraps that are consumed by animals.
Vegan leather
Another environmental concern around leather is the land use and emissions from the animal agriculture
Taking that into account, the Environmental Profit & Loss statement in 2018 claimed that vegan leather production emissions can be up to a third lower than real leather
While that might be the case when looking at the totality of the animal agriculture industry, it doesn’t have the fine detail. If the leather made in the US is from cows used for meat, is the issue leather or meat consumption?
Additionally, one of the main components of vegan leather are plastics, specifically polyurethane and polyvinyl chloride
With an estimated 13 million tonnes of synthetic fibers enter the oceans each year, this presents a very real environmental threat
That’s not to mention the microplastic pollution issue. There were more than 5.25 trillion macro and micro plastic pieces in the oceans in 2023, that’s equivalent to 46,000 pieces per square mile
Now both vegan leather and real leather can be more sustainable. Vegan leather can be made from plant-based materials and real leather can be treated in ways that don’t dump harmful chemicals into wastewater.
Again, for me the key is our fast fashion addiction which i think causes us to not pay as much attention to where and how our clothing is manufactured
For the record, i have the same issue with fake fur. Humane real fur for me is better then fake fur from an environmental perspective.
But i’ll get off my soapbox now and back to the science
I have two more research articles on making leather more sustainable
The first one is from research out of the Kyiv National Unversity of Technology and Design in Ukraine and the University of Antwerp in Belgium
Collagen for the pharmaceutical, biomedical and cosmetic industries is usually derived from fresh animal tissues. As you can imagine, that is not very sustainable. There is now renewed interest in recycling waste streams, getting collagen from waste material and that includes the non-tanned leather waste.
The research team took leather waste from various steps before the tanning process and used acetic acid and NaCL to obtain collagen protein extracts from the waste material.
In order to test the quality of the collagen, they formed it into gels and tried to grow HEK293 cells on it.
Derived from human embryonic kidney cells
Used in a wide range of cellular biology research and to produce therapeutic proteins and to safety test a variety of chemicals
The researchers found that the collagen gel that they produced worked as least as well as traditional bovine collagen for supporting cell growth.
This indicates that this type of collagen derivation would work well for more commercial indications such as use in the medical and cosmetic industries.
Lastly, and probably my favorite, making vegan leather out of mushrooms
Mushrooms are the fleshy, fruiting body of a fungus and mycelium is the root-like structure of a fungus
Mycelium is lightweight and biodegradable and can be grown from waste sources
It has a relatively high strength-to-weight ratio
There seem to be a number of applications for this remarkable biological material intcluding acoustic insulation
One research group out of Korea sought to make a mycelium-based leather from a variety of polyporales fungi (an order of about 1800 species of fungi, most of them rot wood, some are food items or used in traditional Chinese medicine)
The team grew the fungi in the lab, harvested it and turned it into “leather” by soaked the mycelium in glycerol, ethylene glycol and polyethylene glycol and then using corn zein and tannic acid to “cure” it.
They cut samples from the mycelium mat “leather”, to test for different properties
The team found that F. fraxinea was the most suitable strain for mushroom leather production and that using cheesecloth with 20% PEG and hotpress was the method that made leather with a high tensile strength
The team recommended strain hybridization to improve the mushroom leather’s performance
There are a few companies already making mushroom-based leather, including MycoWorks (as featured in the NYTimes) and Mylo Unleather

Glossary:

vascularized/nonvascularized - with or without blood vessels
Keratinocytes - flattened skin cells in the epidermis that produce keratin
Enzymes - proteins that speed up chemical reactions

Cocktail party facts:

Most vegan leather is made from plastics
The ancient Egyptians are credited with perfecting the vegetable tanning of leather
You can use mushrooms to make leather!

I hope you’ve enjoyed this deep dive into leather. Personally, I learned a ton about the process, the pitfalls, and of course mushroom leather!! (Never not going to be cool).

LuxeSci is a production of Erevna Media, produced by me, Dr. Lex. Audio engineering by Dr. Dimos and our theme music is Harlequin Mood by Burdy.

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References

LuxeSci Show Notes: S3E4 - How to Make Leather?

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