In one of my current activities, I have had the chance to work with some folks developing a new medical adhesive. Since sticking things together is at the forefront of my mind these days, I thought I would take this opportunity to briefly share some ways that Biomimics are working on this in the medical field.
First, let’s take a look at the Sandcastle Worm. Also known as the Honeycomb worm (Phragmatopoma californica), it exudes glue used to stick bits of sand and other particulate together in the form of a casing. This glue does not dissolve in water; actually, it has the ability to displace water and adhere to surfaces in water-based solutions and solidifies soon after being secreted.
Dr. Stewart has found two synthetic polymers that imitate the action/reaction of the Sandcastle Worm’s glue. This new glue solidifies as a response to acidity and to temperature where it is liquid at room temperature and solid at body temperature and is twice as strong as the worm’s glue. The team envisions this product as a new means for putting compound/complex fractured bones back together until they can heal on their own.
Larvae make a mobile home
After being inspired to look toward nature for medical adhesives, Dr. Stewart looked to the Caddisfly (Trichoptera) next. The larvae of the fly studied lives underwater and makes itself a mobile home (who would have guessed the first Winnebago was developed 150-200 million years ago!), or a shell, of sand grains and other debris stuck together into a tiny tube shape with spun sticky silk. The researchers say that the larva uses the silk just like “using Scotch tape on the inside of a box to hold it together.”
The silk is extruded from a pair of glands as part of an organ called a spinneret. The adhesive looks like a double ribbon and is akin to silk spun by silkworms and spiders but with an adaptation that acts as the equivalent of underwater acetate tape.
The silk differs from moth and butterfly silks by the use of phosphates to make fibroin proteins and electrical charges that make the silk sticky even under water. Modern latex paint and dental fixture manufacturers have recently (and independently) found the usefulness of phosphates as adhesion promoters.
Caddisflies attach the phosphates to serines thus producing a negatively-charged amino acid. The other amino acids making the silk protein are positively charged. Chains of these proteins are laid out to align the positive and negative charges in a manner that align and attract each other. The plan is to use this technology in surgical situations by creating a “tape” that can bond to a variety of surface conditions in what is virtually the equivalent to an aquatic environment.
Caddisflies have successfully penetrated aquatic habitats around the world ranging from fast moving rivers to stagnant marshes. Their adhesive is able to bond to a large variety of surfaces underwater ranging from hard and soft to organic and inorganic materials.
Protein-based adhesives of mussels
Researchers at universities such as Northwestern and UC Santa Barbara are making great strides in unraveling the remarkable protein-based adhesive (based on the amino acid dopa) of mussels. These proteins cross-link in water creating a solid adhesive plaque that can attach, or stick to, a large variety of substrates. The hope here is to make a series of adhesive hydrogels to be used in biomedical applications for dental industries as well as the reattachment of tendons and ligaments.
Underwater adhesion has been independently evolved by four phyla: caddisflies, sandcastleworms, mussels, and sea cucumbers. In Biomimcry, when you see the same type of evolution independently evolving by different organisms, this is known as a deep principle. Deep principles are typically the best places to begin looking for a solution to the problem at hand.
Geckos can walk across a rough ceiling, and even hang there by one foot without falling! It was discovered at UC Berkeley over a decade ago that this was accomplished using what is the equivalent to millions of frayed hairs so small (called seta with a diameter of 4.2um frayed into thousands of micro-hairs with a diameter of about 200nm) that they use Van der Waals attractive forces (Van der Waals forces are the forces, attractive and/or repulsive, generated between molecules other than those forces caused by covalent bonds or electrostatic forces of ions). The tips of the setae are of a specific shape that allows maximum adhesion when the foot is laid flat but, by rolling the foot forward and increasing the setae shaft angle to 30 degrees or more greatly reduces the surface area and allows the foot to be picked up from the surface they were just adhered to.
Research was continued at MIT with the specific focus of using Gecko technology in a surgical tape (band-aid) made of a strong, water resistant, flexible and biodegradable polymer able to replace cyanoacrylate-based (Krazy Glue) surgical adhesives. The result is a new polymer with its “hairs” created by a mold process using technologies based in the computer chip industry. The tape is scheduled to be available in hospitals in the next five years.
To make this medical tape a reality, multidisciplinary team consisting of experts in nanofabrication, surgeons, biocompatibility experts, polymer chemists and mechanical engineers was required. The gecko technology provides us with another major foundation of Biomimicry: the need for interdisciplinary teams to aptly decipher and develop the technologies provided to us by nature.
The gecko tape craze was also picked up by Northwestern University with an interesting twist. Researchers at the university were not happy with the adhesion capabilities of geckos in aqueous environments so they called upon the mussel to fill the void. Their gecko-mussel combination tape is called “geckel” and utilizes a gecko-type nanostructure with diameters of 400nm coated with a mussel-mimetic adhesive synthetic polymer that improves wet adhesion of the coated nanostructure 15 times over the uncoated nanostructure.
I hope this little adventure into a few of the various ways that nature—from worms to geckos—has provided us with clues on how to create effective adhesives that can bind anatomical structures together long enough to allow natural healing to occur. Although the morsels on employing Biomimcry were brief (deep principles and interdisciplinary teams) they are nevertheless powerful tools for the study of nature as the science of Biomimicry become a strand of glue that binds us to the world we live in.