In The Lab: Synthesising Polymers

In one of the earlier sessions of Exploring Everyday (Bio)Chemistry Society, we discussed the properties of natural polymers (including spider silk). Our focus now moves on to artificial polymers (including rayon and nylon). During our experiment based sessions  of this term, we replicated one of the earliest methods of synthesising rayon (a semi-synthetic fibre designed 125 years ago by the English chemist, Charles Frederick Cross).

First, a solution of tetra-amine-copper(II) ions must be prepared; I measured out 10 grams of basic copper carbonate and, working under a fume hood, added 100 cm³ of 880 ammonia solution. Despite operating in one of the best-ventilated labs, the choking scent of ammonia wrapped its tendrils around my airways almost immediately- suffocating me. I can safely say that the outcome of this experiment was still definitely worth it!

 

 

 

 

 

 

[Carefully measuring out the copper carbonate and dissolving it in ammonia to form a solution of tetra-amine-copper ions]

Next, I added 1.5 grams of finely shredded cotton wool to the solution. The cotton wool provides the cellulose (a natural reactant) for the reaction- which is why rayon is only a semi-synthetic polymer.

 

When cotton is dissolved in this solution, the resulting mixture becomes surprisingly viscous (with an almost shower gel-like consistency).

This initially presented a slight problem in achieving complete dissolution- a problem which was solved with the help of an exciting device I’d never had the chance to use before: a magnetic stirrer.

A magnetic stirrer consists of a small bar magnet which is dropped into the solution and a device which creates a rotating magnetic field – forcing the bar magnet to rotate and, hence, stir the solution.

The resulting effect is that the solution appears to stir itself – much to the shock and delight of some of the younger attendees of the session!

 

 

 

The final step is to simply inject a thin stream of the deep blue solution under the surface of some sulfuric acid.  As the solution enters the acid, its pH drops swiftly and the complex compound is converted into insoluble rayon.

 

Although the rayon initially appears deep blue in colour, as the copper (II) ions diffuse out of the rayon (reacting with the sulfuric acid to form copper sulfate solution), the colour quickly fades away- leaving behind only the delicate, translucent fibres.

              

 

 

Unraveling the Secrets of Spider Silk – 1

Until quite recently, I’ve always considered myself as a bit of an arachnophobe – the very thought of a spider’s unnerving eight-legged dance across the floor is enough to send a tsunami of shivers down my spine.

In the past few months, however- I’ve been forced to interact with spiders (and their handiwork) more than I would have necessarily wanted to. Last November, my cat greeted me with sunlight glinting off the gossamer wisps trailing from her whiskers, purring contently having just devoured another of her newfound culinary delights – spider webs. Although superficially endearing and quirky, the risk of her tucking in to a web coated with the toxin 2-pyrrolidinone (a form of chemical warfare some spiders wield against predatory ants and other creatures that threaten their webs) had compelled me to assess the toxicity of the webs that have accumulated in neglected corners of the garden.

In general, threatening spiders such as the black widow and brown recluse spin unconventional web formations such as the tangle web – infamous for its haphazard arrangement of strong silk threads. Examining the structure of the spider webs my cat gravitates to every morning has allowed me, for the first time, to truly appreciate the intricacies of arachnid architecture. As I retreated back to my office, to a more comfortable distance from the nearest spider, a wave of security washed over me and, intrigued, I started to research the synthesis and spinning of spider silk.

Spider silk is a natural polypeptide which is technically classed as a scleroprotein (a broad category of structural proteins which encompasses collagen (which is found in our ligaments) and keratin (in our nails and hair). There are many different types of spider silk which are chemically designed to suit its purpose; dragline silk, for instance, is used to create the radiating support lines of the web structure and is also used to connect the spider to the web. Swathing silk is a special silk used to wrap and immobilise the spider’s prey. Spiders even create a unique silk for ‘parachuting’ which is released into the air and caught by the wind- allowing the spider to fly!

To enable them to produce so many different types of silk, spiders have a surprisingly complex anatomy; the underside of their abdomen has three to four separate silk glands containing a watery fluid of amino acids known as ‘dope’. Microscopic tubes pump the dope to spinnerets (appendages which solidify and spin the silk). The silk leaves the spider through spigots – protrusions with muscular valves – which control the diameter of the silk fibres. The faster and tighter the strand is drawn, the stronger the silk.

Spigots emitting silk (Gasteracantha sp.), SEM, [Credit: Dennis Kunkel]

Whilst the exact chemical composition of these proteins depends on a combination of genetic and environmental factors, including the species of spider and its diet, the main protein in spider silk is fibroin which consists predominantly of two amino acids: glycine and alanine. The elasticity of spider silk is due to glycine-rich regions of the protein chain which allow the protein to fold in on itself to form a beta-spiral structure.  Capture silk, the most elastic kind of spider silk, has a very high concentration of glycine and so can extend up to 4 times its original length.

 

 

Because of its fantastic properties, it is no surprise that scientists want to harvest spider silk and combine it with other materials to design strong, lightweight bulletproof vests and very flexible, biodegradable rope. The issue is that, unlike silkworms, spiders cannot be farmed to produce large quantities of silk as they are cannibalistic and would resort to eating their neighbours if they were forced into close proximity of each other. Additionally, spider silk is so fine that more than 400 spiders would be needed just to produce a square metre of the material.

Scientists have therefore been trying to recreate spider silk in the lab; chemical synthesis of spider silk is not yet viable since we still do not fully understand the silk structure. Currently, replication of silk is achieved through genetic engineering- silk genes have been inserted into Escherichia coli bacteria to successfully produce the proteins spidroin 1 and spidroin 2. But an unforeseen flaw cropped up; the silk produced by the bacteria was surprisingly weak due to faulty cross-linking  in the protein structure.

The next avenue of research is inserting the silk genes into goats to produce spider silk proteins in their milk since milk production in mammary glands is similar to silk protein production in spiders so proper protein cross-linking could be achieved in goats.

 

 

 

 

 

Visual Prosthesis : Latest Innovations

The face of biotechnology is constantly evolving; blink and you’ll miss it. Latest developments in visual prosthesis are bringing us one step closer to the miracle of restoring sight to the blind.

Over a year ago, I blogged about one of my many visits to the Wellcome Museum in London and, in particular, my interest in the glass eye collection of W. Halford (1870-1920). Curiosity had compelled me to further research the history of ocular prosthetic devices. One of the earliest ocular prostheses dates back to 2900–2800 BCE and serves a purely aesthetic purpose with its radiant “gold lines patterned like sun rays”.

Modern ocular prostheses have become safer and more lifelike. They are typically made of medical grade plastic acrylic in order to avoid issues such as shattering of the prosthetic in the eye. Realism of artificial eyes have been continually developing since the first ocular prosthesis; the main limitation being realistic pupil movement. Ocularists and eye surgeons have always worked together to make artificial eyes look more realistic. For decades, all efforts and investments to improve the appearance of artificial eyes have been dampened by the immobility of the pupil. The most recent solution to this issue is to use an LCD display to vary the pupil size depending on the light intensity of the environment. Despite these large strides forward in the aesthetic qualities and features of ocular prosthetic devices, until very recently, such devices have not offered the most valuable feature of all: vision.

Last night, in the most recent issue of the National Geographic which focuses on the future of medicine, one article in particular caught my eye; researchers have made the possibility of a true bionic eye tangible through 3D printing.

The concept of electrically stimulating the visual cortex and retina to produce artificial sight first surfaced in the 18th century, however (due to technological and anatomical difficulties) this concept is only reaching actualisation within the past decade. The retina  contains light detecting cells called photoreceptors which convert light energy  stimuli into electrical energy for transmission to the brain. In most cases, blindness is caused by the death of these photoreceptors as opposed to the transmission nerve cells that respond to stimuli and send messages to the brain. While those nerve cells cannot detect light on their own, they can respond to electrical stimulation; this is what much of the research into visual prosthesis is focused on.

The Argus II Retinal Prosthesis (patented in 2011) is the first of such devices to receive FDA approval; it consists of a camera mounted onto eyeglass frames which is connected to a processor and retinal impant containing 60 miniscule electrodes. Although certainly a blessing to patients who have been able to regain sight with this prosthesis, the Argus II is far from a natural bionic eye.

              

 

The  latest reported prototype of the bionic eye (developed by researchers from the University of Minnesota in August 2018) has an uncanny realism and functionality that I found entirely fascinating. The bionic eye has a 25% efficiency in converting light into electrical signals thanks to its semiconducting polymer photodiodes which cover a 3D printed base of silver particles lining the interior of the hemispherical glass dome. This prototype represents a milestone in the field; it proves that semiconducting materials can be ‘printed’ onto curved surfaces and, more importantly, that such devices can be cost-effective- designed by a humble 3D printer in under a hour as opposed to in high-tech microfabrication labs.

Beautiful as it is, this bionic eye still needs further development to make the surface softer and more suitable for implantation and to improve light conversion efficiency. Within another decade, such devices should be available as true cures to blindness- perhaps the future isn’t so dark after all…