An MD&DI February 1998 Column
R&D HORIZONSBiochemists are teaming up with marine biologists to examine an unlikely source of superadhesive technology.
Formulating the right adhesive for a particular medical device is a tricky affair, constrained only by the laws of chemistry and the expertise of the maker. Suppliers have literally thousands of possibilities at their fingertips, including epoxy resins, silicones, cyanoacrylates, and urethanes in a range of viscosities, strengths, and hardnesses. These adhesives, which must hold a USP Class VI rating to pass muster with FDA, might be cured thermally or by electron beam, visible light, or ultraviolet light. They might glue catheters or syringes, pacemakers or angioplasty balloons. Or they might bond the components inside electronic implantables.
Researchers hope to unravel the secret behind the byssus threads that help mussels adhere to underwater surfaces. Photo courtesy of J. Herbert Waite, PhD.
"If we look through all our products and can't find one that fits the particular need," says Paul Atkinson, senior engineer at Ablestik (Rancho Dominguez, CA), "we set loose our chemists to formulate one."
It is, essentially, R&D on the fly, with adhesives tailored to fit the specific product. Ablestik's specialty is putting together the electronics that run implantable medical devices. Chemists there alter the characteristics of adhesives by changing the type of filler or resin. More dramatic effects are obtained by changing the catalyst. "It is very much like baking a cake," notes Atkinson. "You change some of the components and it changes the flavor dramatically."
High-tech improvements are possible as well. Researchers at Tennessee's Oak Ridge Center for Composites Manufacturing Technology (CCMT) have devised a way to inexpensively alter epoxy resins so they can be cured very rapidly using an electron beam. "Historically, there have been few, if any, suitable epoxy materials out there that were E-beam curable," says Chris J. Janke, principal investigator for CCMT. "We have now found an effective way of modifying these materials." The technology could be used to create materials that equal or exceed heat-cured epoxies in performance, yet neither require long cure times, consume large quantities of energy, nor involve the expensive tooling needed for heat curing.
Still, the myriad choices of adhesives and the iterative enhancements in their formulations can obscure the fact that the current state of the art is sorely lacking. "No one is ever really happy with adhesives," says John M. Questel, president of Adhesive Consultants (Akron, OH), a testing laboratory that specializes in medical adhesives.
Several years ago, a major medical manufacturer sought advice from Questel and his crew regarding aortal balloons, which were still in the development phase. The company needed to attach a Mylar balloon to a polyurethane tube. The balloon had to withstand pressure of about 250 psi in order to clear an obstructed artery. But pumping the balloon up caused it to blow off its mount. "If it pops off in an artery, it sooner or later gets to the brain and kills the patient," Questel says. His team was eventually able to find the right structural adhesive but only after exhaustive research.
An electron micrograph shows the dark granular filaments that hold the byssus of the zebra mussel to the substratum. Photo courtesy of Thomas Bonner, PhD.
Questel comments that balloon pressure ratings are determined largely by the strength of the adhesive. Such limitations are common in the medical device industry, where device specifications are matched to the capabilities of adhesives. Yet adhesives are still favored over other bonding methods. "Many disposable medical devices are now using adhesives for assembly purposes because they are the most economical way to do it," says Questel. "Even though the adhesive is about $1000 a gallon, it is very economical because one drop is all you need to assemble a catheter."
Ideally, new adhesives would offer the advantages of the current breed and greater strength at the same time. That may be possible, but chemists may have to dispense with the known laws of chemistry and embrace the still unknown laws of nature.
COCKLES AND MUSSELS
"It is downright embarrassing that some sailors make their livings chipping barnacles off ship bottoms and those bonds are made to pretty dirty surfaces under salty, biochemically active water," says Robert Baier, PhD, professor and director of the Industry/University Center for Biosurfaces at the State University of New York (SUNY) Buffalo. "You would think after all these centuries of chipping them away, we would have learned how to copy them to at least hold two parts of a pacemaker together."
Baier is among a clutch of biochemists at institutions across the United States who believe future successes in medical adhesives will spring from knowledge gained from barnacles, mussels, and algae. These unassuming creatures have the remarkable ability to stick themselves in the midst of rushing water on the propellers and hulls of ocean-going ships, at the mouths of huge water intake pipes, and along the water line of supertankers, "where the detachment forces are most ferocious," as Baier notes.
The glues that hold these very different animals in place are virtually identical, he says. Nature has devised a method for producing this ultimate superglue and conserved it across species. If chemists could make a synthetic version, it might be the ideal bonding material for medical devices designed to be placed inside the human body, which is essentially a bag of water. The problem is that attempts at reverse engineering have failed to reveal what nature has evolved over the past several millions of years. "We have not come to understand the complexity of what occurs in natural systems," Baier says.
Baier has been able to glean, however, that the system appears to be similar, at least generally, to a two-part epoxy, comprising a thin coat of biofilmthe slime that naturally grows on submerged surfacesand a secretion from the animal. The foot of the mussel, for example, releases an exudate that is basically a complex phenolic compound that interacts with the biofilm on the underwater surface to form an adhesive.
There are two major barriers to synthesizing this exudate. One is finding out how to add the requisite second hydroxyl group to the benzene ring, which together form the phenolan amino acid called tyrosine. "Nature somehow makes the extra hydroxyl group on that tyrosine outside the cell," Baier says. "We don't know how that is being done." The second barrier is learning how to cross-link the compound to provide strength against shearing.
Baier and his SUNY colleagues may be near conquering the first barrierthe oxidation of the tyrosine to form a synthetic compound similar to the mussel exudate. They have programmed bacteria to synthesize a major portion of this compound. But there is a problem. "Even if we could make a quart jar of the mussel adhesive cement in the lab, the chance of getting it to cross-link and harden like an epoxy cement is very low right now because we haven't yet discovered how the cross-linking takes place in nature," Baier says. The answer appears to involve enzymes that catalyze this molecule into a sticky form.
An understanding of this transformation might come from transmission electron micrographs being taken by Thomas Bonner, PhD, professor of biology at SUNY Brockport. Bonner's micrographs show the adhesive as granules of glue resting beside granules of the enzyme, a type of catechol oxidase. Evidence suggests that these granules open up and merge. An understanding of how this process occurs may come from a detailed analysis of the extraordinarily complex structure of these granules, as well as from studies of material called byssus secreted by the foot of the mussel.
"Electron micrography allows us to see the way the byssus is organized at a high level of magnification," Bonner says. "Then we get to look at the interface between the byssus and the substrate to understand the roles they play."
The byssus, explains Bonner, is outside the living cells that compose the animal's foot, and is the mussel's equivalent of fingernails. Both the byssus and human fingernails are composed of protein. But unlike fingernails, the byssus forms tethers, called byssus threads. The adhesive connects the byssus to the underwater surface, and the byssus tethers the mussel to the surface.
Studies at SUNY Brockport indicate that after the granules become part of the byssus and participate in the attachment processthe actual gluingthey are transformed into a structure composed of very densely packed filaments. Still evolving is an understanding of the role played by the biofilm that coats the surface being attached. This slime is made of living organisms, primarily bacteria, some algae, and perhaps fungi, along with decomposing organic material.
An electron micrograph shows the dark interactive surface of a natural animal glue. Photo courtesy of T. Bonner.
"There is preliminary evidence that the byssus is initially released in a semi-liquid or gel state, which then flows into the interstices of the biofilm and solidifies, trapping parts of the biofilm in the byssus itself," Bonner explains. "The reason we think this is that we find a fair number of bacteria trapped in the bottom of the bys-sus where it interacts with the surface."
Bonner is studying the zebra mussel, best known for its propensity to clog water intakes for power plants and water treatment plants along the Great Lakes. Marine animals such as the New England blue mussel also secrete a byssus and threads, but in these animals the threads are elastic and serve to absorb the force of the water as it rushes past. These threads elongate and then recoil to their original position when the force is removed. This natural "shock absorber" enhances the animals' ability to adhere to the surface and may provide a model for developers of medical adhesives.
The connector between the two surfaces might be a monolayerrather than a long tethercomposed of some attachment factor, suggests J. Herbert Waite, PhD, a professor of marine biology and biochemistry and joint professor of chemistry and biochemistry at the University of Delaware College of Marine Studies (Newark). "Here you would not necessarily have to worry about curing the adhesives; you could exploit the fact that the adhesive can stick opportunistically to any hard surface."
Waite is dissecting natural adhesives into their tiniest components, while search-ing for the underlying rules that will make sense of the data being uncovered in his laboratory. Unfortunately, performing reverse engineering is not a viable option. "Doing so assumes that these organisms subscribe to our rules," Waite explains. "Since nature has shown time and time again that it is capable of defining its own rules, the approach is fraught with uncertainty."
It is abundantly clear that the chemical functional groups on the macromolecules created by these animals are not present in any of the current generation of manmade adhesives, Waite says. That in it-self is a great motivator to push on with the research.
One of the great mysteries driving Waite involves the cross-linking that occurs within the adhesive, specifically the way this protein folds in upon itself. Current chemical understanding dictates that protein folding occurs in the middle of a molecule, which would draw the material together, minimizing its interface with the surface and, consequently, reducing adherence. But this natural superglue does the opposite. It spreads out, maximizing surface-to-volume ratio. "It certainly is not doing what other proteins are doing," Waite says.
The explanation will depend on coming up with new rules to explain the behavior of this protein. Waite is confident that the rules can be determined without rewriting current principles of organic chemistry. The research now being conducted will eventually produce effective models to explain the actions of these macromolecules. "This will allow people to make effective adhesive molecules, as well as the enzymes needed to synthesize them," Waite adds.
Producing these natural superglues in bulk might be a great help to the medical device industry. Researchers in this field might learn enough from the natural processes to build a "peptide maker" that could produce quantities of these glues on demand. Among the leading candidates are yeast and virally infected insect cell lines. The infected insect cells intrigue Waite the most because they have a lot of the enzymes that appear to be needed to do the processing that gives the adhesive proteins their requisite "stickiness."
Exactly when such molecular production lines will be available, however, is difficult to predict. Waite believes science may be able to imitate the adhesion of marine invertebrates within 10 years. "But that may not be the direction we want to go," he says. "When you characterize a very complex strategy, you have hurdles that you overcome along the way. The proteins and partial strategies that come from these can frequently have very useful spin-offs that can be applied without knowing the whole answer."