About Natural Materials
This page aims to provide the reader with insights into the breadth of natural materials, their use and their properties. Contributions will be made by Board Members of the Natural Materials Association, and by invitation to experts within the natural materials sectors. The page will be updated over the course of the coming months. Until then, we hope you enjoy reading the articles we put up.
CHITOSAN: A CHITIN DERIVED BIOMATERIAL
Dr Parvez Alam, FIMMM, FRSB, School of Engineering, The University of Edinburgh, UK
Originally discovered in mushrooms, chitin is the second most abundant biopolymer on earth. It is preponderant in the exoskeletons of arthropods such as crabs, shrimps, arachnids, insects and scorpions, in the cell walls (hyphae) of fungi, and in the radulae of molluscs. Chitin is a structural polysaccharide (based on sugars) with broad application potential in a range of industrial sectors. Chitosan, a derivative of chitin, is increasingly being used in the biomedical and biotechnology sectors. This brief article discusses the extraction of chitin and its chemical conversion to a commonly used derivative, chitosan. The article will furthermore detail the application potential of chitosan as a biomaterial.
Extracting chitin and its conversion to chitosan
Chitin is typically extracted from the exoskeletal waste of arthropods, primarily the shells of edible shrimps and crustaceans. The shells are comprised primarily of chitin, calcium carbonate and proteins. To extract solely the chitin, the exoskeletons are initially washed and dried, after which they are subjected to mechanical grinding. Following this, calcium carbonate is removed via a demineralisation process (specif. decalcification) by heating in 1 Molar (1M) sodium hydroxide for up to 72 hours. Proteins are then removed using a hot acidification treatment in hydrochloric acid for up to 48 hours to leave pure chitin. At this stage, the chitin is still pigmented (by mainly carotenoids) and is bleached using e.g. a potassium manganite/oxalic acid mixture to remove the pigments. Finally, the N-deacetylation of the chitin gives rise to chitosan, Figure 1.
Figure 1. Process for extracting chitin from waste arthropod exoskeletons. After grinding, decalcification, deproteination and decolourisation, chitin can be deacetylated to chitosan, a useful biomedical material (Parvez Alam, CC-BY-NC-SA 4.0 – Natural Materials Association).
General biomedical applications of chitosan
Chitosan is an excellent biomaterial as it readily protonates in neutral solution, meaning it is water-soluble and has a high affinity to negatively charged surfaces (e.g. the mucous membranes found in the body). It is biocompatible and has the additional benefits of being biodegradable and easily chemically functionalised. As such, chitosan can be used in a number of biomedical areas including; bioadhesion, drug delivery, tissue culturing, tissue regeneration (as a haemostatic agent), antimicrobials, and bioimaging (as a labelled agent), Figure 2. However, despite these fantastic properties, chitosan films are brittle, which can make them harder to use as a biomaterial. Improving the fragile nature of chitosan films is therefore a current key challenge facing biomaterials researchers. One promising mechanism to mitigate chitosan’s inherent mechanical weakness is through the development of hybrid materials comprising chitosan blended together with other polymers. The following sections provide examples of two major areas within which chitosan has biomedical relevance; wound healing and drug delivery.
Figure 2. Biomedical uses of chitosan, (Parvez Alam, CC-BY-NC-SA 4.0 – Natural Materials Association).
Wound healing is a specific biological process related to the general phenomenon of growth and tissue regeneration. The wound healing process consists of five stages involving complex biochemical and cellular process; homeostasis, inflammation, migration, proliferation, and maturation. Nanofibrillar chitosan is often used within cross-linked polymer hydrogels to improve the process of wound healing. Such hydrogels interlink polymer chains to form three-dimensional networks. The cross-linkers form either covalent or ionic bonds and are typically lower in molecular weight than the linking polymers. The final properties of cross-linked hydrogels will depend on the extensiveness of cross-linking (i.e. the cross-link density). Chitosan hydrogels are essentially structures of chitosan-chitosan cross-links, hybrid polymer cross-linked networks, or ionic cross-linked polymer networks. These have several benefits for the wounded area including: the ability to decrease inflammation by absorption of fluid from the inflammation site, the blocking of nerve endings to reduce pain, the enabling of nucleation sites for healthy cell growth (via scaffolding), antimicrobial properties, and an ability to act as a temporary glue to effectively hold the site of damage together.
Figure 3. The mechanisms of chitosan-based hydrogels to promote wound healing. From RSC Adv., 2018, 8, 7533-7549, CC-BY 3.0.
Nano-chitin fibrils are also beneficial in wound healing. The repeating monomer subunit of chitin, N-acetylglucosamine (NAG) is understood to originate and cross-link to collagen in a wound. Both nano-chitin and nano-chitosan can be degraded by lysozymes in the fluid of the wound. Chitin has a mechanical benefit over chitosan in wound healing as it increases the tensile strength of the wound to a greater degree.
Chitosan has distinctive features, which makes it a suitable candidate for controlled drug delivery. These include biodegradability, nontoxicity, and biocompatibility with the human body, which are conjointly important, as degraded chitosan should not cause any inflammatory response in the body. An example for drug encapsulation within chitosan is via a self-assembly process, Figure 4. Chitosan is cationic and therefore naturally hydrophilic. As such, hydrophobic moieties grafted along chitosan backbones will easily cling to hydrophobic drugs within a hydrophilic solvent, essentially turning the chitosan backbones into envelopes that contain the drugs. This self-assembly process is one of many variants, each of which relies on the judicious manipulation of positive and negative charges within molecular systems.
Figure 4. Drug entrapment into hydrophobic-functionalised chitosan self-assembled nanoparticles. From Polymers 2018, 10(3), 235, CC-BY 4.0.
Positive/negative charge relationships can also be used to deliver drugs from the nano-chitosan vessels to specific sites of interaction. While physical changes to the nanoparticle such as erosion, diffusion and swelling may result in the release of drug from a chitosan vessel, physicochemical mechanisms are slightly more targeted to substrates. Figure 5 provides a step-by-step example of how positively charged chitosan nanoparticles might adhere electrostatically to negatively charged mucosal layers, after which physical processes such as diffusion or erosion, take over to enable drug release over time.
Figure 5. Schematic representation of chitosan loaded nanoparticles (CS-NP) structure and interaction with the mucus layer. From left to right: CS-NP upon reaching the mucosal layer bind to the negatively charged mucus by virtue of electrostatic attraction and release the drug over time. Pharmaceutics 2017, 9(4), 53, CC-BY 4.0.
NANOCELLULOSE: A POTENTIAL MATERIAL FOR SUSTAINABLE PRODUCTS
Dr Dipa Roy, School of Engineering, The University of Edinburgh, UK
What is Cellulose?
Cellulose is the most abundantly available organic macromolecule on Earth. Cellulose is found in different sources like wood, agricultural biomass, sea animals, algae, and fungi. It is the main compound found in plant cell walls which helps the plant to remain stiff and strong.
Figure 1: Cellulose in plants (redrawn after Carbohydrate Polymers, 90, 2012, 735).
The chemical structure of cellulose resembles that of starch, but unlike starch, cellulose is extremely rigid. This rigidity imparts great strength to the plant body and protection to the interior of plant cells. Cellulose is 1, 4 linkage of beta glucose monomers, as shown in Figure 1. It is, therefore, a polysaccharide (Latin for “many sugars”). Several polysaccharide chains remain arranged in parallel arrays to form cellulose microfibrils. The individual polysaccharide chains are strongly held together within the microfibrils via hydrogen bonds. Due to the presence of hydrogen bonds, they are crystalline in nature and are rigid, strong and stiff. The microfibrils are bundled together to form cellulose macrofibrils. Cellulose fibrils remain embedded in an amorphous matrix which comprises of mostly lignin and hemicellulose (Figure 2).
Figure 2: Cellulose fibrils embedded in an amorphous matrix of hemicellulose and lignin (Redrawn after ACS Sustainable Chem. Eng. 2018, 6, 2807),
There are several techniques available which can separate out the crystalline cellulose from the non-crystalline, amorphous matrix and make them available for various applications. Microcrystalline or nanocrystalline cellulose, extracted from various cellulosic resources, can be used as a valuable material in developing sustainable products.
What is Nanocellulose?
With the advent of nanoscience, researchers have focused on producing nanocellulose by breaking down the structure of cellulose microfibrils using various mechanical, chemical or enzymatic techniques (Figure 3). Cellulose nanofibrils or nanoparticles are generated by mechanical or chemical treatment. Mechanical treatments include high-pressure refining, grinding or high-pressure homogenization. Acid hydrolysis is the most commonly used chemical treatment. During acid hydrolysis, hydrolytic cleavage of the glycosidic bonds take place mainly in the amorphous regions of the cellulose, releasing individual crystallites.
Figure 3: Extraction of nanocellulose.
The increase in the nanocellulose research lies in their interesting properties such as low density, low cost and high mechanical properties. Cellulose in its nanocrystalline form has a very high tensile strength, high Young’s modulus and is a very good reinforcing filler for various composite materials. Nanocellulose can be obtained in various shapes and sizes depending on the type of treatment. They are called by different names, as given below in Table 1. However, the surface of cellulose/nanocellulose are highly hydrophilic as they have many hydroxyl groups in their chemical structure. This hydrophilicity makes them prone to moisture absorption. Various chemical modifications are carried out, if required, in order to tailor their moisture absorption behaviour to suit diverse range of applications. This also increases their compatibility with less hydrophilic materials.
Table 1: Different Forms of Nanocellulose (Angew. Chem. Int. Ed. 2011, 50(24), 5438; Composites: Part A, 2016, 830 19).
*Bacterial, or microbial, cellulose has different properties from plant cellulose and is characterized by high purity, strength and increased water holding ability.
Nanocellulose from Agrowaste
The original article can be found in Ind. Eng. Chem. Res. 2011, 50, 871–876.
Apart from various plant resources, agricultural waste can be used as a resource for the extraction of nanocellulose. The utilisation of agrowaste, such as rice husk, rice straw, sesame husk, sugarcane bagasse, potato peel, pineapple leaf fibre etc. can offer the benefit of low cost. Below is an example where sesame husks have been used as a resource to extract nanocellulose (Figure 4).
Figure 4: Sesame husk converted to nanocellulose powder.
The microscopic images of the sesame husk and the extracted cellulose whiskers are shown in Figure 5.
Figure 5: Sesame husk to cellulose whiskers (microscopic images).
The cellulose whiskers when subjected to further mechanical treatment produces cellulose nanoparticles, as shown in Figure 6.
Figure 6 Cellulose whiskers converted to cellulose nanoparticles.
Use of natural materials can lead to greener and low cost sustainable products that reduce environmental burden, as these materials can be safely returned to the natural carbon cycle by simple biodegradation in a composting environment. Cellulose, being strong and stiff, has the potential to be used in a diverse range of applications. Cellulose is a cheap and abundant natural resource which are key requirements for industrial application. Nanocellulose, having a smaller size and higher surface area, can impart interesting properties to products. It is used in various applications such as nanocomposites, smart coatings, paper and paper board, as thickeners, flavour carriers, and suspension stabilizers in food, medical, cosmetic and pharmaceuticals. However, a uniform and steady supply chain, and variabilities which might arise in the nanocellulose properties due to different sources, must be kept in consideration for successful application in larger scale. Further research can open up new avenues to use this ‘gift of nature’ in a more resourceful way.
REVOLUTIONARY MATERIALS: MYCELIUM
Issi Rousseva, Architect
What is Mycelium?
The word mycelium literally means “more than one”. It is actually a plural form of the word Mycelia. The word has New Latin and Greek origins and was first coined in text in the early 1800’s, and refers to the thread-like body of a fungus. Long before trees overtook the land, earth was covered by giant mushrooms. Researchers found that land plants had evolved on earth about 700million years ago and land fungi by about 1,300 million years ago. The largest living organism in the world today is a honey fungus measuring 2.4 miles (3.8km) across in the Blue Mountains in Oregon. Fungi live everywhere that moisture is present and play an important role in energy cycling within, and between, ecosystems. Fungi are found in terrestrial, marine and freshwater environments, and are part of a diverse community of “decomposers” that break down dead plants and animals. Mycelium is the vegetative part of a fungus, consisting of a network of fine white filaments (hyphae). In simple terms, it’s the fungus that mushrooms are made of, essentially, the term mycelium is used to refer to those thread-like structures of fungi. Mycelium (plural mycelia) develops from the fungal hyphae.
How Does Mycelium Form?
Mushrooms do not reproduce by seed or gather energy through photosynthesis like plants do. They reproduce by means of spores. These spores germinate to produce a mass of interwoven, single-cell wide structures known as hyphae. Collectively, masses of hyphae are known as the mycelium. These multicellular structures can grow into macro-size structures, which we most often recognise as mushrooms.
Figure: Mycelium (photo from Biohm)
How does Mycelium Grow and How Can it’s Shape be Manipulated?
Fungus absorbs nutrients from its environment (substrate, log, etc.) through its mycelium, in a two-stage process. First, the hyphae secrete digestive enzymes into the decaying wood or other complex organic compound substrate. These enzymes break down biological polymers into smaller units such as monomers, creating new molecules (these molecules are now soluble nutrients, such as simple sugars, nitrates and phosphates). The mycelium then absorbs these monomers, using a combination of facilitated diffusion and active transport. This consumption of organic and synthetic waste causes it to grow- the shapes it grows into can be manipulated, as can its properties, through the different types of waste it is paired with.
As the mycelium grows, it begins to resemble a dense network of long, microscopic fibres that grow through the substrate. Once the mycelium has fully built its network, it transitions to the next stage; building a mushroom and this is where humans can intervene. The mycelium can be coaxed with supreme precision into complex structures so small, they’re invisible to the naked eye; this is done by manipulating and controlling the temperature, CO2, humidity and airflow- all of which influence the growth of tissue. This is a rapid process and the accumulation of fibres becomes a visible speck after a few hours and a visible sheet of material after a day or two.
Figure: Mycelium panels (photo from Biohm)
What Potential does Mycelium have?
Biohm is a UK based company who work with Mycelium, as well as other alternative natural products, which they are currently testing for different markets. Biohm believe the fungi Mycelium is an extraordinary organism with significant untapped potential. They are currently working with over 300 different strains of mycelium to create sustainable alternatives to some of the construction industries most damaging materials. Biohm’s development behind Mycelium is through allowing nature to lead innovation in the construction industry, supporting cradle to cradle principles and the circular economy. Their range of products are completely natural, and can be considered as multi-tasking with a view to replacing current carbon-heavy materials in the construction industry. Mycelium is a great alternative for those due to its carbon-negative properties which are as a result of manipulation of naturally occurring biological processes. This could be a significant breakthrough in the global challenge of ridding the planet of synthetic waste.
Figure: Mycelium tiles (photo from Biohm)
Application of Mycelium and it’s Eco Credentials
Mycelium’s thermal, mechanical and physical properties are currently being explored, however Biohm have claimed they already know the material is capable of achieving higher insulation values than current premium alternatives and it has good mechanical and physical properties too. The following information is from Biohm’s product testing of mycelium. Applications of mycelium range from thermal insulation and acoustic panels to interior architecture applications and in product and furniture design (as are currently being explored by Biohm in the UK).
Figures: Mycelium insulation panel (photo from Biohm) (left), Mycelium acoustic panels (photo from Biohm) (right)
Properties and Benefits of Mycelium
The material is capable of achieving higher insulation values than current premium alternatives and has promising structural integrity. The tests performed so far have been on Biohm’s bottom-of-the-line mycelium samples and produced values as low as 0.024 W(m*k), however their market ready premium product is expected to exceed the current thermal properties by 3-14 times.
Mycelium is naturally self-extinguishing when exposed to a fire and almost forms a complete fire barrier.
Mycelium is capable of coping with any level of moisture or humidity due to its ability to naturally act like a wick, expelling excess moisture from itself (as long as it is not exposed to prolonged submersion). In addition, during the curing phase of production, the panels naturally get coated in a thick Mycelium skin, which acts as a barrier and allows the material to be even more water resistant.
Health and wellbeing
As a natural occurring material, Mycelium releases minimal, if any VOCs and as a breathable material, allows for air and moisture flow which can help minimise damp in buildings
Biohm’s mycelium insulation is grown on agricultura or food waste (a critical waste stream in the UK with 10.2bn tonnes produced annually). It can be custom grown, avoiding the need for on-site trimming.
Recyclable & cold-compostable
At the end of its life, the material can be fed back into the production process, eliminating waste entirely. The material will also naturally decompose offering pH balanced nutrition to soils.
Existing Mycelium Projects
Mycelium has a wide range of applications and the following example projects from around the world illustrate this.
* Italian architect Carlo Ratti has grown a series of arched architectural structures from mushroom Mycelium.
Figure: Arched mycelium structures. Photo by Marco Beck Peccoz.
* London-based designer Nir Meiri has created a series of table lamps using mushroom mycelium as an alternative to synthetic materials.
Figure: Mycelium table lamps. Photo from Biohm.
* Redhouse Architecture is looking at recycling derelict homes by demolishing them, combining the waste with mushroom mycelium and then using it to build new, biodegradable structures.
Figure: Mycelium used in architectural structures. Image by Redhouse Architecture.
* The British furniture maker Sebastian Cox is the latest designer to work with mushroom mycelium. He has teamed up with researcher Ninela Ivanova to investigate the material’s potential in commercial furniture design, creating suede- like designs.
Figure: Mycelium furniture. Photo by Petr Krejci.
* A group of Indian and Italian architects have built a pavilion in southwest India using mushroom mycelium.
Figure: Mycelium pavilion in India. Photos by Krishna and Govind Raja.
* Architect Dirk Hebel and engineer Philippe Block have started exploring with using fungi to build self-supporting structures. According to the duo, mycelium could provide the structure of a two-storey building, if it is designed with the right geometries.
Figure: Mycelium used to build self-supporting structures. Photos from Dezeen
* Dutch textile designer Aniela Hoitink has created a dress using disc-shaped pieces of mushroom mycelium, which she believes will "change the way we use textiles".
Figure: Mycelium textile design. Photos from Dezeen.
* Dutch designer Eric Klarenbeek has 3D-printed a chair using living fungus, which then grows inside the structure to give it strength. The chair is the result of a collaboration between Klarenbeek and scientists at the University of Wageningen to develop a new way of printing with living organisms.
Figure: Mycelium 3D-printed chair. Photos from Dezeen
* New York studio The Living, created MoMA PS1’s gallery pavilion in 2014 – the structure was made up of a cluster of circular towers built from bricks that were grown from corn stalks and mycelium.
Figure: Gallery pavilion made from mycelium bricks. Photos by Kris Graves.
The Future of Mycelium
With the current global issues of global warming, deforestation and destruction of natural habitats, natural materials as alternatives to unsustainable, high-embodied-energy materials, could be the answer that everyone is looking for. A study recently published in the Proceedings of the National Academy of Sciences (PNAS) found that at least 88 percent of the Earth's ocean surface is polluted with plastic debris. This is having a significant detrimental impact on our wildlife, particularly our marine species. However, according to Biohm and scientists at Kew Gardens in London, not only can fungi be used to create sustainable building materials, it can also be used to break down waste plastic, helping to clear the deluge of mass-produced plastic waste covering the earth and affecting already-fragile habitats. This alone illustrates mycelium’s huge potential as a commercialised, mainstream material.
MASTER CRAFTSMEN OF THE KALAHARI
Dr Parvez Alam FIMMM, FRSB, School of Engineering, The University of Edinburgh, UK
The original article can be found in "Materials World vol. 27 Issue 12", the official magazine of the IOM3.
The San people of Southern Africa are considered by many to be the most ancient race of people in the world. They are nomadic, egalitarian, hunter-gatherers, many of whom have maintained much of their lifestyle, despite widespread marginalisation of their communities in Southern Africa. Divided into three main nations (the !Kung, the Tuu and the Tshu-Khwe) and further into a plethora of groups/sub-groups, here, we focus on the Ju|’hoansi San people, Figure 1, a Southern !Kung group located in the Kalahari desert (North-West Namibia/North-East Botswana) and comprising circa 1400 people living in 36 N!oresi (villages). The Ju|’hoansi are one of the few San groups who are still able to practice their hunter-gatherer lifestyle, though the breadth and freedom of their practices have been considerably limited by law.
Figure 1. A Ju|’hoansi family living in the Kalahari desert of North-Western Namibia.
Tools of the Ju|’hoansi people
The tools of the Ju|’hoansi can be divided into three broad categories; those used for gathering, general everyday tools, and hunting tools. Gathering cloaks (Kaross) (Figure 2) are typically made from animal skins originating from large antelopes such as eland or kudu. Sharpened wooden tools, or combined wood-iron tools, are usually used for digging for foods and other useful materials. General tools may include fire-hardened woods to create a pestle and mortar, quivers made of fire-hardened hollowed tree roots (Figure 2), and male-female sticks for starting fires. The Ju|’hoansi hunting tools (bow and arrow, and spear) are perhaps the most technically significant, exhibiting careful and well-planned engineering designs. This article will focus on the assembly-procedures, materials selection, and geometrical designs of the Ju|’hoansi arrows (Figure 2) and is based on research insights developed by The Materials Modelling and Design team at The University of Edinburgh.
Figure 2. Hollowed out tree root to make a quiver (top left), an example of an arrow (top right) and a gathering cloak, or Kaross (bottom).
Ju|’hoansi arrows – materials and manufacture
The Ju|’hoansi have invested considerable time and effort into developing the composites and adhesives that leads to the manufacture of each arrow. So much so, that each stage of manufacturing is critical for a successful final product, which typically takes three days from start to finish. The process is naturally slow since there are curing times for adhesives, speciality methods of preparing arrowhead parts, and each procedure is completed by hand. Figure 3 provides examples of arrowheads made of root (Rhus tenuinervis), shinbone (of giraffe) and ductile iron (a material introduced during the German occupation of 1884), each of which is inserted into a hollow reed (Phragmites sp.). With the exception of these arrowheads, the majority of other materials used to construct the arrows are common to them all. This includes the reed (Phragmites sp.) forming both the reed shaft (all arrows) and the reed link shaft, a thinner reed connecting the bone-to-bone and metal-to-bone in the bone and metal-tip arrows. Also included are two separate types of glue, one of which is viscous amber sap from the trunk of the tree Terminalia sericea, while the other is a composite comprising milky latex exude from the roots of the Ozoroa schinzii bush mixed with the fine black ash of freshly burnt Aristida adscensionis grass. Giraffe and kudu tendons (from the Achilles) are also used in the making of these arrows, and each arrowhead is finally tipped with a deadly poison usually extracted from the grub of Chrysomelid (leaf) beetles (Diamphidia sp. and Polyclada sp.). These grubs are typically dug out from the base of Commiphora sp. or Sclerocarya birrea trees, at about a foot below the surface of the ground. It is generally understood, that the chronological order of materials used to construct arrowheads began with the root, was followed by bone, and ended with the ductile iron. Nevertheless, all three arrowheads are still in use by the Ju|’hoansi, the root arrowheads being preferred for the close range hunting of small sized prey, while the bone and metal arrowheads are used to hunt larger prey at longer ranges.
Figure 3. A Rhus tenuinervis root arrowhead (top) giraffe shinbone arrowheads (middle three) ductile iron arrowheads (bottom two)
Ju|’hoansi arrows – design
The Ju|’hoansi arrows are for the most part, composites made up of natural materials. Reeds (Phragmites sp.) form the main shaft in all arrow types. The cellulose fibres of these reeds grow unidirectionally along the length of the reed. This means the reeds are of high stiffness and strength along their lengths but are weak in their transverse directions. Since the arrow heads are wider than the reeds, and are inserted and firmly fixed into them, the reeds will fail between the fibres through forced transverse contact with the arrowhead. To circumvent this problem, the Ju|’hoansi reinforce the portion of the reed that will bear the transverse forces exerted by inserting the arrowhead. They reinforce the reed by first wetting the carbonised black Ozoroa schinzii glue and then applying it to the surface of the region for reinforcing. Following this, they tear thin strips (approximately 0.5mm thick × 1-2mm wide) of water saturated giraffe tendon, which they wind around the reed over the surface of the Ozoroa schinzii glue. They conduct the same procedure for the reed link shaft observed in the bone-tip and metal-tip arrowheads (cf. Figure 3). Once dried, the glue fixes the tendon tightly to the surface of the reed shaft/link shaft, and the tail end of the arrowhead is covered in Terminalia sericea glue after which it is inserted firmly into the reed. In the cases of the bone-tip and metal-tip arrowheads, the same procedure takes place with the appropriate ends of the arrowhead parts being glued and inserted into the reed link shaft as well.
Figure 4 (a) Cutting of the Ozoroa schinzii root, which after heating expels a milky latex that is subsequently mixed together with (b) ashes of Aristida adscensionis grasses (c) the finished latex/ash adhesive is shown attached to the stick at the top, while the adhesive attached to the lower stick is the amber sap of the Terminalia sericea tree (d) an the root of a Rhus tenuinervis plant is cut to a balanced arrowhead while still wet.
Figure 5. Application of the latex/ash adhesive by wetting and rubbing to the surface of Phragmites sp. reeds before giraffe tendon is wrapped transversely around the reed to create an affixed composite reinforcement (left) giraffe shinbone is cut to a sharp point before it is inserted into the Phragmites sp. reed link shaft –amber sap adhesive from the Terminalia sericea tree is used to glue the arrowhead to the inside of the reed link shaft.
To balance the weight of the arrowheads from tip to shaft, the Ju|’hoansi counterweight their arrowheads. This decreases tilting problems when the arrows are fired. In the case of root-tip arrows, loss of material towards the tip of the arrowhead is balanced by preserving a thick section of the root at the shaft-end of the arrowhead. The procedure is slightly more complicated for the bone-tip and metal-tip arrowheads, as the counterweights are essentially, large pieces of giraffe shinbone, with slightly more bulbous geometries than the arrowhead tips. These counterweights are chaffered down at the ends to fit into both the reed shaft and the reed link shaft. The tips of these types of arrows are more finely constructed, both being long and thin in comparison to the bulbous counterweights. Wet giraffe shinbone based tips are manufactured by cutting and grinding, while ductile iron is heated over coals and hammered, cut and ground into shape. The Ju|’hoansi typically use metal iron fencing that they find in their localities. Interestingly, the geometries of the arrowhead tips are widely varied and this is understood to relate to the density of the material used to make the tip. The root tips, which are the largest, are also of the lowest density (0.9g/cc), while the smaller bone tips are higher density (2.0g/cc) they are not as dense as the ductile iron tips (7.6g/cc), which are geometrically, the smallest of the three arrowhead types. This indicates that the Ju|’hoansi arrows are geometrically designed as well as manufactured, as a response to their material and physical properties.
We may further note that there is a design logic in how the reed link shaft is used. It would not for example, be any more time consuming to create a single part arrowhead in the bone-tip and metal-tip arrows, in similitude to the root-tip arrow. Yet, the Ju|’hoansi preferentially design these in three separate parts, the central part being a composite joint made of giraffe tendon reinforced reed. Following discussions conducted with the elders of a Ju|’hoansi family, we understood that these were in fact purposely made to break. Whereas a root arrow is a short-range projectile that can easily be recovered, the longer range of use of the bone-tip and metal-tip arrows means that these arrows can be easily lost, either in a distant hunting ground, or while attached to a fleeing animal. Arrows are vital tools for hunting and their reuse and recyclability can save precious time and natural resources. As such, the Ju|’hoansi design the arrow-tips of the bone-tip and metal-tip arrows in such a way that they will break off after impacting and penetrating an animal. Animals such as giraffe or eland are large and the poison takes time to spread. While the Ju|’hoansi track their kill (which may take up to 3 days for large, strong mammals), they can retrieve the majority of the rest of their arrow, which will typically break off at the reed link shaft where the hunt took place. This allows the Ju|’hoansi the opportunity to reuse much of the arrow, only having to replace the reed link shaft and the tip, should the tip not be found with the dead animal.
Today, the San people’s hunter-gatherer lifestyle is critically under threat. They have been marginalised to the extent that many San groups now accept handouts of game that has been hunted by foreigners. Anthropologists predict the San way of life has less than 25 years before it ceases to exist altogether. The Ju|’hoansi San are living models of humanity’s history yet, very little is known about the design parameters they have manipulated to satisfy the specific functional requirements of their hunting tools. Their arrows are exemplars of function-specific engineering and design, are manufactured using simplistic tools, and are made primarily of natural materials sourced from the local environment. They have been designed to be balanced in flight, lightweight and aerodynamic, to be recoverable and reusable, and to be damage tolerant. The Ju|’hoansi are the composite arrow master crafters of the Kalahari, and have developed their technologies over 200,000 years of trial and error.
Dr Parvez Alam FIMMM, FRSB, School of Engineering, The University of Edinburgh, UK
The original article can be found in "Materials World vol. 27 Issue 7", the official magazine of the IOM3.
Biological adhesions between bacterial cellulose nanofibrils can improve the performance of biomedical skins and grafts.
Bacterial cellulose (BC) discovered by A J Brown in 1886 is somewhat similar to plant cellulose. It has the same molecular formula, but is chemically more pure as it is free from other plant polysaccharides such as hemicellulose and lignin. It has a higher tensile strength, can be grown into almost any shape, has a superior water retention capacity, and a considerably finer architecture.
Properties such as these have made bacterial cellulose an ideal ingredient in foods such as nata de coco, a sweet jelly with increased fibrousness, chewiness and texture through the addition of bacterial cellulose. It has also been used as a thickener and stabiliser to help retain the viscosity of softer foods.
More recently, the properties of BC have gained the attention of materials scientists in the biomedical sectors where it is used as a wound dressing material, in the pulp and paper industries where it is used as an ultra-stiff paper base, and in coatings industries, where it can be used to stabilise coating suspensions.
BC is sourced naturally in South East Asia from microorganisms such as Acetobacter xyllinum, to cater for the multifaceted Asian food industries. It is a renewable, sustainable and biodegradable nanofibril, and has a nanoscale filamentous structure with very high crystallinity and outstanding properties of stiffness ranging on average between 78-143GPa. Separating the nanofibrils of BC is far simpler and less process-intensive than it is for plant-derived cellulose nanofibrils, making it an attractive ‘green nanomaterial’ with considerable potential in biomaterials applications.
The bulk matter of BC is effectively networked by β conform hydrogen bonds, which help align cellulose molecules into its tight crystalline structure. The β-conformation, as it is known, is a straight-backed molecular chain. The straightness of the molecules allows them to pack densely together, which can improve molecule to molecule interactions through hydrogen bonding. Therefore, BC surfaces are dense with hydrogen bonding sites, however the stiffness of the individual nanofibrils makes it difficult for interfibrilar alignment. BC nanofibrils can therefore create a weak network continuum when packed together with other BC nanofibrils, however, the true mechanical potential of BC is never reached in this way. This is reflected in the stiffness of BC sheets, which is reported to be between 2-15GPa, and is therefore a factor of 10 lower than the nanofibrillar stiffness. How can we scale up BC in a way that reduces losses from its nanofibrillar stiffness? Natural bio-adhesives may have some of the answers.
For materials scientists, the natural world is a constant source of design inspiration. Topics such as structural hierarchy, adhesion and material morphology are of particular interest, since they elucidate novel design guidelines in the advancement of mechanical materials. Biological adhesion is ubiquitous in the natural world, where it exists in a plethora of different function- specific chemical forms.
The design and utility of bio-adhesives in natural materials technologies also constitute an environmentally responsible engineering practice. Synthetic adhesives that could be used to connect BC nanofibrils are often toxic to the environment and do not easily degrade. This is why, when incredible examples of stickiness are found in the natural world - for example, aggregate and flagelliform silks, extracellular polymeric substances (EPS) from biofouling diatoms and mussels - attempts should be made to mimic at least their function and design strategies by using similar, naturally sourced bio-adhesives, preferably with minimal chemical modification.
When bio-adhesive functions are considered, it is possible to some degree to simplify their mechanisms of adhesion to those dominated by intermolecular secondary forces such as electrostatic and van der Waals, and those dominated by off axis sidechains that obstruct intermolecular shear. A good example of the former can be observed in the nanocrystals of structural biological silks. The stiffness of such nanocrystals is derived primarily from a large concentration of hydrogen bonds that arrange in a quasi-planar manner between β-sheet layers within the nanocrystal. The result is a nanostructure with an exceptionally high stiffness for a polymer.
Sidechains described in the latter case are different, as secondary interactions that arise between side chains and molecules tend to be off-axis, and therefore less likely to have the same cumulative level of quasi-planar resistance described in the first example.
In the case of molecules such as chitin, this has been shown to be beneficial to intermolecular adhesion, as the acetyl sidechains laterally stabilise the crystalline form of chitin, resulting in both higher stiffness and fracture toughness than in deacetylated chitin, chitosan, which lacks the side chains. Both of the mechanisms of adhesion described in the examples above can significantly improve the mechanical properties of biopolymers. It would stand to reason, therefore, that it is possible to improve the properties of ‘other’ materials by mimicking similar functional designs.
The materials design team at The University of Edinburgh focuses on biological and biomimetic composites design. They employed both of the bio-adhesive mechanisms previously described to improve the adhesion strength between BC nanofibrils. The specific amino acid monomers alanine and glycine were chosen as they are known to enable hydrogen bonding dominated stickiness in materials such as silk.
In the first instance, the amino acids were used as free-moving secondary bond-forming bio-adhesives between BC nanofibrils. During sheet-forming, the amino acid monomers were able to energetically optimise their locations and orientations between the BC nanofibrils, and in doing so, migrate into the most stable electrostatic attachment. The team also grafted the same amino acids to BC surfaces via esterification reaction on TEMPO-oxidised BC nanofibrils. TEMPO is a free radical reagent used to oxidise primary alcohols aldehydes.
The TEMPO-oxidisation reaction uses 2,2,6,6- tetramethylpiperidine-1-oxyl radicals to oxidise the C6 primary hydroxyl on the cellulose chains to form C6 carboxylate groups. This then allows for the grafting of alanine or glycine as sidechains to the surface of the BC nanofibril. Unlike the free-moving amino acids, when amino acids are grafted to BC surfaces in this way, they are constrained at one end, creating a hairy surface at the molecular-level.
Using experimental tests on manufactured BC sheets using both methods described, coupled with molecular dynamics simulations, the team learned that free-moving amino acids favour adhering BC surfaces together by aligning themselves parallel with the BC. Amino acids covalently attached to the TEMPO-oxidised BC surfaces, on the other hand, are considerably more constrained. The limited mobility in such instances, forces them to electrostatically bond to the nearest available hydrogen-bonding sites.
As such, the mobility of the free amino acid monomers gives them an advantage over pinned sidechains, raising the stiffness of BC sheets to more than 100% that of unglued BC sheets. The sheets made in the laboratory are about the size of a person’s hand – bigger sheets can be made by scaling up the manufacturing technology, which already exists, so sheet size depends solely on the size of the machine.
The researchers also found TEMPO-oxidation and amino acid grafting of BC gives rise to about 50% improvement in stiffness. Clearly, the free movement of bio-adhesives trumps molecular pinned sidechains in this instance, and if BC sheets are to go closer to their theoretical maximum (nanofibrillar) stiffness, bio-adhesives are a good starting point. Considerably more research needs to be done to tighten that hydrogen bonding network between BC nanofibrils to reach their theoretical maximum stiffness. In the future, clever molecular design strategies will need to be applied to bio-adhesive technologies, with the aim of maximising the potential for strong secondary interactions.
Figure caption: Free amino acids, as seen on the left hand side of the figure, are able to position themselves in the most energetically favourable way and are able to attach to cellulose molecules with more hydrogen bonds (red dotted lines) than the amino acids that are grafted to the TEMPO oxidised cellulose molecules, as seen on the right hand side of the figure.
Next, this research will be taken to a more applied environment by engaging industrial support from the paper, board, packaging, and biomedical engineering sectors. The future of TEMPO-oxidised BC needs to be revisited using a variety of different bioglues and bonding methods, given that an end goal is to mechanically enhance BC. The design strategies of molecular materials should be guided by atomistic models, and the most promising of them, chemically replicated and tested.
This research approach is believed to elucidate the larger spectrum of possibilities for the control of the mechanical behavior of BC materials.
Bacterial cellulose is a natural, non-toxic, biodegradable material with exceptional mechanical properties and characteristics. It is, however, a nanomaterial and as such must be conjoined with others of its kind to form larger bodies of engineering material. The properties of the nanofibrils are reduced when they are collected together into larger structures and as such a challenge for materials researchers is to deduce novel methods for combining them without reducing their green-materials character. Bioadhesives are a material choice that potentialises the recovery of nanofibrillar mechanical character by improving stress transfer between the nanofibrils. Free-moving bioglues and bioglues pinned through TEMPO-oxidation show promise in the development of stiffer bacterial cellulose materials, however, research efforts in this area are still sparse and applied research will be needed to control and manipulate their mechanical properties for specific applications.
BAMBOO HYBRIDS CAN BOLSTER UK TIMBER SUPPLIES
Hector Archila, University of the West of England, and Founder, Amphibia BASE Ltd.
The original article can be found in "Materials World", the official magazine of the IOM3, HERE.
A relatively common material is not being embraced fully for building supplies.
Interest in bamboo is growing, through research and media campaigning about the material’s environmental benefits, properties and its ability to replace wood, steel and carbon-fibre. But bamboo is not commonly used in high-performance applications or permanent structures, and its potential in construction remains untapped.
Academic and industrial research carried out in the UK aimed to address the challenges associated with using bamboo in durable, safe and cost-efficient structures. Further combining the grass with UK-grown wood could maximise use of this material and make economic sense for the domestic forest sector.
Figure caption: A 2,000m² warehouse using Guadua in Bogotá DC, Colombia. All images credited to: Hector Archila.
High bamboo scaffoldings used when working on Hong Kong’s skyscrapers, and earthquake and typhoon-proof structures in Ecuador, Colombia and in the Philippines are a testament to the material’s mechanical properties. While regarding bamboo as a green steel and drawing a like-by-like replacement for steel is fundamentally wrong, as I said in the previous work Bamboo reinforced concrete: a critical review on the website Open Access, July 2018, the axial stiffness-to-weight ratio of bamboo is remarkably high and almost the same as that achieved for steel (25 x 106m2s2).
However, this only applies to the mechanical properties of bamboo in the direction of its fibres i.e. it is longitudinal only. In any other direction, radially or tangentially, bamboo is very weak and tends to split and crush, unlike steel, which is isotropic. This is not to say bamboo should be scrapped, but more carefully considered.
Bamboo has a higher CO2 absorption than other materials. Compared with a native British tree like Scots Pine, Guadua – a giant species
of a tropical ‘woody’ bamboo endemic to South America – is able to absorb about five times more CO2 in a given period.
The mean yield class for Scots Pine in the UK, as given by the Forestry Commission, is an increment of 10m3 per hectare per year, while for Guadua it is 45m3 per hectare per year. Scots Pine would reach maturity at around 40 years old, while Guadua would be ready to harvest aged between three-five years.
Bamboo could provide large quantities of feedstock material for construction products that retain the fixed CO2 for long periods of time, for example in buildings where an effective lifespan ranges between 50-100 years. A way to unlock bamboo’s potential is by combining it with complementary materials like wood, which can address its radial weakness and thin-walled section, while exploiting its high axial stiffness and maintaining its high carbon and environmental credentials.
Based on research carried out at the University of Bath, UK, Amphibia BASE is working to commercialise bamboo and demonstrate the business case for structural engineered bamboo products in the UK and worldwide. The key to making this a reality has been the use of thermo-hydro-mechanical (THM) technologies, which rapidly and efficiently transform bamboo into flat planks with higher density and improved mechanical properties.
THM technologies involve temperature, moisture content and mechanical pressure controls to modify the material’s properties, for example its density and hardness, or to shape it into the desired form. Processing can be finished in 15 minutes, compared with three hours by conventional means. The company’s THM process reduces the void spaces within bamboo conductive tissues, bringing the naturally scattered bamboo fibres closer together without causing cell damage – effectively densifying bamboo and producing a more homogeneous cross-section. This is evident from microscopy analysis of the material before and after THM densification. These are desirable features in structural engineering when designing and predicting the behaviour of materials in high-performance applications.
As a result of the THM process, the density and Young’s modulus of bamboo are both increased almost two-fold from 540–890kg/cm3 and 16.88-30.72GPa – the force applied in kilonewton per square millimetre – respectively. This straightforward densification process, reduces waste by 50%, as well as the high energy and labour intensity of conventional manufacturing processes of engineered bamboo products (EBPs) for non-structural applications. When laminating the THM densified bamboo, glue use is reduced from 30–3% per overall weight of the final product in some commercially available EBPs. These features were demonstrated in cross-laminated bamboo products G-XLam, manufactured by Amphibia BASE in the UK, at lab-scale.
However, when densified bamboo is laminated on its own, the resulting product is very stiff, heavy, over-engineered and costly. The bending modulus of G-XLam 3 – three layers – obtained through testing was 23.68GPa, twice the value for the same property in cross-laminated timber (CLT) panels with the same three layers 11.6GPa. CLT is becoming increasingly popular for use in buildings reaching heights once thought unviable with timber. For instance, an 85.4m-high, all-timber tower designed by Voll Arkitekter was recently finished in Brumunddal, Norway, and is currently the highest tall timber building in the world. Overall, G-XLam panels have one-fifth of the thickness of CLT panels and double the density. By contrast, when a lower density timber is used, the number of laminations and product density can be reduced to an acceptable and cost-efficient level while retaining the stiffness and stability of the laminated product. Therefore, THM-densified bamboo can be used to strengthen low-density and low-strength wood, adding value to the final product and reducing the overall section of the structural timber product. This can also be applied to other low-density cores forming the hybrid composite.
In the case of wood, statistics from 2018 by the Forestry Commission show about 70% of the sawn wood and 100% of the plywood and structural timber products used in construction in the UK were imported mainly from Europe. This is partly due to the low density, poor finishing and unsuitability of UK-grown wood, it being used mostly in low value-added applications, for example fencing. Timber experts may argue that in the UK, we need to loosen strength-grading standards for homegrown timber and establish a robust capacity to dry the wood for high value-added products such as CLT.
Around 50% of the forested areas of the UK is Sitka spruce. Due to the UK climate, Sitka grows very quickly, which does not allow for tissue consolidation and renders a low strength (C16) and lightweight timber that is commonly considered low-quality and value. However, when combining UK timber of lower strength class with THM-densified bamboo of higher stiffness, the resulting hybrid product can achieve properties equal to those achieved using timber from mainland Europe, for example in CLT panels. Although UK forests have recovered since the industrial revolution, there is limited land, thus the available timber.
Wood-bamboo hybrids could significantly reduce the amount of wood required for engineered products and tackle quality and strength issues in UK-grown supplies. Further benefits include the support of the local UK forest industry and rural economy, the reduction of the UK’s reliance on imported European timber and CLT elements, and the development of new markets locally and internationally. Also, exploitation of under-utilised and readily available bamboo resources in tropical countries using state-of-the-art technologies developed in the UK can offer a cost-effective and highly sustainable alternative to carbon and energy-intense conventional materials used in construction, such as steel and cement.
Technological challenges regarding adhesion and drying of home-grown wood to acceptable levels, which are key to the success of this commercial venture, are being addressed. Additionally, the use of the vast knowledge and expertise on timber for building applications helps tackle the current lack of internationally recognised standards for standalone structural bamboo products. Currently, funding and investment is being sought to develop a pilot facility that rolls out the technology and showcases the viability of producing wood-bamboo hybrids in the UK.
SPIDERS 'GO WITH THE FLOW'
Dr Chris Holland, Department of Materials Science and Engineering, University of Sheffield, UK
Figure caption: The golden orb-weaving spider Nephila edulis
When asked by my Dphil supervisor if I would like to look into silk rheology I didn’t know what rheology was and how it would shape my research career. Five years on when asked what I do for a living I now reply with enthusiasm “I study how spiders and silkworms spin silk”.
Rheology concerns the study of the flow and deformation of matter. Here I will discuss how we have found use for this technology to study features of the natural world, specifically in understanding how Nature produces one of her finest materials, silk. More usual for this technique are questions about engineering materials such as the visco-elastic properties of paints, polymers and food.
Silk is a biological material, a protein fibre produced through a process called spinning. It is unique to the arthropods, where it is wide spread. The most famous silk producers are the spider and many moths, foremost among them the silkworm. However it may surprise you that silk is spun by some bee’s and ants too! The evolutionary success of silk can be attributed to its versatility, being used for protection from predators to capturing of prey with reproduction and dispersal of young in-between. Spiders are exceptional in producing more than one type of silk. Indeed they are able to make up to seven different types of silk, with each fibre possessing mechanical properties specifically tuned to their biological function.
Whilst silk is clearly an attractive material in nature man has also been captured by its qualities. The silk of the Chinese silkworm (Bombyx mori) is the world’s oldest commercial fibre, dating back over 5000 years. Being one of only two truly domesticated insects (bees being the other) today Bombyx are reared in the millions and current worldwide production of silk is in excess of 100,000 metric tonnes with a value of over $1bn (before the added value of high-end textiles). However the reasons for the commercial success of silk go way beyond simple aesthetics. Spider silk in particular is weight for weight stronger than steel and it is much tougher than Kevlar. It could be considered the pinnacle of any fibre production process and quite unlike its industrial counterparts silk is ‘green’, being produced sustainably at room temperatures, using only water as a solvent and being completely biodegradable.
Figure caption: The Chinese silkworm Bombyx mori
Yet despite our fascination with this supermaterial for five millennia we know surprisingly little about its evolutionary origins and synthesis. However because silk has been selected primarily for its physical properties and to perform outside the body, it is possible to translate the testing methodologies we use to characterise our man-made materials to try to understand why silk is so interesting and important.
One such technique applied to silk has been straightforward tensile testing. By stretching silk fibres we are able to determine many of their physical properties, from how stiff they are to how much energy they can absorb (toughness). The most informative animal to use in this approach is the multi-talented spider. By carefully ‘silking’ the spider onto a reel we are able to control the production rate (reeling speed) of a silk and in conjunction with varying the temperature and humidity it is possible to produce silks from the same feedstock with a wide range of mechanical properties. By repeating this for different spiders we begin to see overlaps in these properties, to the extent that even silks with different amino acid compositions can be tuned to have almost identical material properties. This implies that the processing of a silk is as important as the feedstock in defining a silk fibre’s performance.
Figure caption: Spiders can produce up to seven different types of silk each with mechanical properties optimised for their usage
Therefore a pertinent question to ask is how is silk spun? Quite unlike other biological materials which are grown slowly over months and years (like hair bone and feathers), silk is by definition spun, in seconds transforming from gel to fibre. In the spider and silkworm the silk protein feedstock is stored as a gel, potentially for extended periods of time, in specialised organs called silk glands. Upon spinning these proteins travel down an elongated tapering tubular duct (shaped like an industrial hyperbolic dye) during which they undergo a transition into a solid fibre.
Examining these events on molecular scale reveals that in order to spin you need a source of energy. This energy fuels the removal of water from the silk proteins and the subsequent alignment and formation of a network of strong amide-amide intra and intermolecular hydrogen bonds. This network consists of ordered (high H-bonding) and disordered (low H-bonding) regions. Whilst it may be thought that a fully ordered crystalline material may be preferable, it is this careful balance of order and disorder at the nano-meter level that is responsible for silk’s superior mechanical properties. When silk deformed (i.e. stretched) stress is concentrated in the ordered regions (the stiffest parts) and then quickly and efficiently transferred to the disordered regions which are able to dissipate this excess energy as heat. Because silk is excellent at dissipating energy it is very tough, making it the ideal material to absorb impact (from a fly hitting a web to a bird attempting to break a cocoon).
But what is the main source of energy that fuels the transition from gel to fibre? Here we draw inspiration from an unlikely source: fishermen. Prior to the introduction of nylon fishing lines in the second half of last century fishermen took the silk glands of Bombyx and repeatedly drew them through their teeth, much like pulling chewing gum. If sufficiently skilled the fishermen could produce fibres that were very strong and in fact superior to the first plastic alternative lines that came onto the market. We now know that quite like the spider and the silkworm, the fishermen were using the same method of introducing energy into silk, by shearing the silk proteins. Here we can use rheology to further our understanding. By characterising exactly how these materials respond to shear deformation (mechanical energy input), we can begin to determine how silk is spun. Through rheology, the study of flow and deformation of matter, we are able to apply these shearing conditions to silk in vitro, thus providing us with a window into the silk production process.
Figure caption: Viscosity shear rate curves of the spider and silkworm reveal they are very similar and can be compared to polymer models, however they are completely different to the same amount of reconstituted (artificial) silk.
Whilst fortunately my research does not involve me pulling silk trough my teeth, there have been moments where studying this material has felt like pulling teeth.
One such moment arose at the start of this work: We quickly discovered that preparing unspun silk for rheological testing requires incredible care and attention. Unspun silk is maintained at the precipice of the transition from gel to fibre, so much so that the smallest introduction of energy initiates this process. Thus when handling these materials the slightest shearing of the material through pinch of the forceps or accidental bump leads to massive variations in the data (over two orders of magnitude for viscosity). This can essentially mask any intended induced experimental variation. Thus during the first year I had to develop new ways of handling silk feedstocks and loading them onto the rheometer before we were able to even start investigating them.
In practice rheological testing involves placing the fresh silk feedstock between two metal plates (8mm in diameter) and moving the top relative to the bottom, thus subjecting the sample to different combinations of stress and strain. From this we are able to get information regarding the materials ability to handle energy over different timescales (oscillation tests) or the degree of intermolecular associations (viscosity tests).
Once the methodology was mastered we were keen to examine differences between the flow properties of silk feedstocks. Given tens of thousands of silks to choose form we confined our initial studies to comparing the silks of the golden orb weaving spider (Nephila edulis) and the commercial Chinese silkworm (Bombyx mori). Our work confirmed previous rheological studies on silkworm dope in that it is a weak gel with Non-Newtonian properties, therefore what about the spider?
The results were quite surprising. We found that despite hundreds of millions of years of evolutionary separation, using unrelated proteins and being selected to produce materials for completely different purposes (protection, silkworm, and capture, spider) the rheological properties of these two silks were practically identical. From a biological perspective this is important as it is a perfect example of evolutionary convergence, and something that is as yet invisible to molecular approaches. Hence we concluded that in order for Nature to spin a high performance fibre these feedstocks are under evolutionary constraints to have a particular set of rheological properties.
Most surprising however was not that these two materials behaved the same as one another, but that their rheological properties were akin to classic molten polymers! This has opened the door to using tools and techniques developed for the polymer sciences to be adapted to study silk. On the flip-side, because of a similar approach to processing our materials we can now learn from silk to improve our current polymeric fibre production methods.
Hence biotechnologically, the spinning pathway of the spider and silkworm is a highly advanced fibre production system. Through it the animal is able to control the energetically efficient production of fibres, with a wide variation of mechanical properties, using the same feedstock, by just altering the spinning conditions. These properties, perhaps even above the high performance characteristics, makes silk such a highly desirable material.
Therefore it is not surprising that many attempts have been made to recreate silk. Typically such artificial silk fibres are spun from natural silk that is “reconstituted” (chemically broken down) and then respun using processing conditions ranging from classical alcohol baths to electrospinning. However the tensile mechanical properties of these reconstituted fibres pale in comparison to their natural progenitors. Thus we were faced by a problem; by testing the properties of the end product alone it becomes difficult to determine what requires improvement, the feedstock or the processing.
Here we took the evolutionary constraints from our first study and turned them into design criteria for artificial silks. Our hypothesis was simple, if you could match the rheological properties of the reconstituted silk feedstock to the native feedstock then you should be able to process them like a silk into a silk-like fibre. Thus our rheological characterisation methods would help us to determine whether reconstituted silk dope actually has the potential (or not) to form a fibre with the qualities of its natural predecessor and model.
Consequently in our second study we compared the rheological properties of artificial and natural silk feedstocks over a wide range of concentrations (0.1-30% dry weight). Our findings were astonishing. For the same amount of silk protein present in the feedstocks there was over five orders of magnitude - a 10000 fold - difference between native and reconstituted silks in their ability to absorb energy (as defined by the plateau modulus) and the strength of their intermolecular associations (as defined by the zero shear viscosity). Put in real world terms the viscosity differences are akin to comparing lard to that of olive oil. We concluded that the act of reconstitution severely damages the silk proteins and that it will not be possible to spin biomimetically until the quality of the feedstock can be improved although rheology now holds the key to unlocking both the secrets of the feedstock and as a QA tool to improve our methods of reconstitution.
Where are we headed in the future? I have shown that rheology has created two new pathways for the field of studying silk, understanding the processing and development of a next generation artificial silk feedstock. From a scientific perspective we must start integrating new tools with rheology to characterise the responses of the silk proteins to shear on a molecular scale. One such approach is our work with the ISIS pulsed Neutron source at the Rutherford Appleton Laboratories where we are now beginning to combine state-of-the-art rheometry with small angle neutron scattering to understand how the conformation of the silk proteins change as they are being spun. From an industrial perspective things are already moving quickly in interesting directions demonstrating silk is more than just a fibre. Because silk can be chemically “unspun” back into a feedstock is may be reprocessed in new and exciting ways, creating structures from films to foams. The initial use of these materials is found in biomedical devices and implants that make use of silks excellent mechanical properties, biocompatibility and its ability to be chemically functionalised.
And what is the final lesson: stop during your spring-cleaning and spare a thought for the spider and its amazing technology.
PROTECTING VACCINES WITH A SUSTAINABLE 'SMART FIBRE'
Angela Morris – CEO The Wool Packaging Company