In the search for new biomaterials, scientists are learning to emulate the attributes of animal and plant organisms
From sea mussels and geckos to spiders and silk worms, an array of animal species are inspiring scientists to develop new medical device materials to solve a host of age-old maladies, such as osteoarthritis, hernias, and various wounds. Responding to the urgent need for biocompatible, bioregenerative, and long-lasting materials to meet the medical challenges posed by an aging population, experts in such fields as materials science and biomedical engineering are turning to biomimetics.
Literally meaning the "mimicry of life," biomimetics is the study of the formation, structure, or function of biologically produced substances and materials such as enzymes or silk and biological mechanisms and processes such as protein synthesis or photosynthesis. Its purpose is to synthesize products by artificial mechanisms that mimic natural ones. Relying on biomimetics as a key weapon in their conceptual armory, researchers are developing products that emulate phenomena from the animal and plant kingdoms, including adhesives, cartilage, and bone.
The Messersmith Research Group at Northwestern University (Evanston, IL; http://biomaterials.bme.northwestern.edu) sees its mission as utilizing biologically inspired strategies to develop new biomaterials for the repair, replacement, or augmentation of human tissue. To that end, the team conducts research into the natural adhesive capabilities of such organisms as mussels, which cling to rocks in tide pools, and geckos, which can scale vertical surfaces and hang upside down from ceilings.
"My research group focuses on the development of medical adhesives and coating materials that prevent fouling by proteins, cells, and bacteria," remarks Phillip Messersmith, professor of biomedical engineering at Northwestern and the research group's team leader. "The idea behind sea mussel-inspired materials is to take advantage of what is known to be very elegant solutions that mussels have evolved for solving problems of adhesion under wet conditions, which cannot be easily accomplished using existing man-made materials." Understanding that adhesives do not work well on wet surfaces or under wet conditions, the group studies the composition and properties of animal proteins with adhesive properties. Its goal is to develop synthetic materials—primarily polymers—that take advantage of the key components found in these proteins that can enhance the ability of the polymers to adhere to surfaces.
For example, mussels and other marine organisms secrete protein-based adhesive materials, enabling them to attach to surfaces. Secreted as fluids, these protein adhesives undergo a cross-linking, or hardening, reaction, leading to the formation of a solid adhesive plaque. A structural feature of mussel adhesive proteins is L-3,4-dihydroxyphenylalanine (DOPA), an amino acid that is believed to be responsible for these adhesive and cross-linking characteristics. Evidence suggests that bulk oxidation of DOPA residues leads to intermolecular cross-linking of the plaque proteins, solidifying the adhesive, whereas interfacial adhesion to substrates is generally believed to be caused by chemical interactions between the unoxidized catechol form of DOPA and functional groups at the surface of the solid substrate.
Engaged in research that could eventually result in novel medical adhesives, Messersmith's group is developing synthetic polymers that mimic the composition and properties of the adhesive proteins found in nature, including those in mussels. Thus, the group employs molecular-level adhesion experiments to understand DOPA's role in biological adhesion and uses this information to design new DOPA-containing macromolecular biomaterials.
While some scientists think that native proteins can be mass-produced and used as adhesives, Messersmith and his team do not embrace that concept, particularly for medical applications. "We believe that native proteins are very immunogenic and will not be well tolerated in the human body," Messersmith notes. "Instead of extracting and purifying native proteins, our approach is to take advantage of key elements of these proteins." Thus, it is known that the mussel's adhesive proteins have unusual amino acids that are very rare in other proteins and are believed to be present for the purpose of enhancing adhesion. Using synthetic polymer chemistry, the Messersmith group incorporates those amino acids, or mimetic versions of them, into new synthetic polymers that can confer adhesive properties similar to those of native proteins. "Our strategy is not one of taking the native proteins and trying to use them, but rather—based on what we know about the native proteins—taking key elements and incorporating them through chemistry into man-made materials," Messersmith says.
Taking advantage of the wet adhesive chemistries that such organisms as mussels have evolved is relevant for medical device applications in, on, or within the body, Messersmith stresses. "In biological fluids, you're always dealing with the presence of water. In any kind of medical or surgical procedure—if you break through the skin—you're going to be dealing with a wet surface. Thus, it makes sense to approach these problems from the perspective of a biologically inspired wet adhesive." Potential applications for such adhesives include bonding two tissues together to repair a traumatic injury or attaching a medical device to a tissue surface.
Most of the liquid adhesives and sealants that the Messersmith group is pursuing for medical applications fit in the category of injectable biomaterials for minimally invasive therapy because they begin as liquids and solidify through a rapid chemical reaction. "Thus, such adhesives have the physical properties of pretty much any type of very fast reacting glue that solidifies from a liquid precursor," Messersmith says.
From the Tide Pool to the OR
Used in such applications as hernia repair, Nerites's adhesive-coated synthetic mesh attached to bovine pericardium is tested for lap shear strength—the force required to pull adhesive-coated mesh away from tissue.
It's a long, hard slog from researching the properties of animals such as mussels to transforming the findings into real-world medical applications, but some biomedical enterprises are, in fact, developing products based on the principles of biomimetics. One such company is Nerites (Madison, WI; www.nerites.com), whose motto is "Inspired by the sea. Perfected by Science." Dedicated to aiding tissue repair, speeding surgical times, and reducing postoperative complications, the supplier provides surgical adhesive and coating products to close internal tears and voids, affix synthetic and biological devices, and prevent bacterial biofilm formation and postsurgical adhesions.
Nerites, which collaborates with the Messersmith group and other university research teams, is also interested in mussels. "Sea mussels are capable of tenaciously adhering to surfaces under saline conditions akin to those in the body," remarks Jediah White, the company's senior director of business development. "Internal adhesives today require dry surgical fields when they are applied, making them difficult to use and occasionally ineffective." In contrast, Nerites's adhesives, which are completely synthetic and based on the adhesive component of sea mussel dihydroxyphenyl proteins, tolerate less-than-ideal settings for application, according to White.
Utilizing a straightforward synthetic chemistry approach, the company fabricates synthetic mimetics of mussel proteins by attaching an adhesive end-group—variations on dihydroxyphenyl groups—to a biocompatible backbone such as polyethylene glycol. The technology can be configured in various ways, White explains. It can be supplied as stand-alone products or as components of other medical devices. For example, it can be dispensed in the form of thin adhesive films for gluing tissue planes together, or it can be incorporated into biological or synthetic meshes to replace sutures and staples as methods of fixation. And by densely packing chains of polyethylene glycol together, it can also be used to create antifouling coatings that stick to implant surfaces and resist bacterial attachment. Nerites' polymers are assembled via cross-linking, yielding cohesion and adhesive strength. By changing the underlying materials and the cross-linking structure, the company can tune an adhesive's functional characteristics for different applications.
While the adhesive has been used successfully in animal models, it remains in the preclinical stage. "Our thin-film technology is a breakthrough that differentiates Nerites from other adhesive companies and enables us to address medical indications where existing adhesives have failed," White insists. "One such application is hernia repair, our lead internal project. An adhesive-coated mesh for hernia repair that obviates the need for sutures or surgical tacks will reduce postoperative pain, speed surgical times, and reduce overall procedure costs."
Of Spiders and Silkworms
Orthox's meniscal cartilage repair device is a tissue scaffold that resembles the human protein fibronectin.
“Biomimetics allows us to learn tricks from nature and find solutions to problems we have,” comments Nick Skaer, CEO of Orthox Ltd (Abingdon, UK; www.orthox.co.uk). Associated with Oxford University (UK; www.ox.ac.uk) and supported by an award from the Wellcome Trust (London; www.wellcome.ac.uk), Orthox develops materials for regenerating damaged cartilage and bone tissues, employing a technology based on a protein found in natural silk fibers. Incorporated into its Spidrex tissue scaffolds, these proteins create a material with a porous structure that supports human cell growth while gradually being absorbed and replaced by regenerating cartilage or bone tissue. As an R&D provider, the company aims to market meniscal repair devices within the next three years based on the material it is developing or supply the material itself to OEMs for other orthopedic applications.
Orthox's biomimetic model is the spider and its unique method for spinning silk. "We're interested in spider silk because it combines different properties," Skaer notes. "First and foremost, it possesses mechanical attributes—in a nutshell, tensile strength, which is approximately five times that of high-tensile steel but combined with significant elasticity. You can extend it by 150% before it breaks. And that makes it one of the toughest fibers known to man." That, according to Skaer, makes it extremely interesting from an orthopedic perspective, where doctors replace skeletal tissues with a mechanical function.
Another quality of spider silk is its protein structure, which strongly resembles the structural properties of a human protein called fibronectin that is found abundantly in the extracellular matrix in cartilage and bone tissue. "So we have a protein in spider silk that orthopedic cells can grow on that also has the potential to emulate many of the structural and mechanical properties that orthopedic tissue possesses," Skaer says. "That's why we're so interested in spider silk as a material for replacing orthopedic tissue—cartilage and bone."
The two scientists that established the technology now being developed by Orthox, Fritz Vollrath from the Oxford Silk Group in the zoology department at Oxford University and David Knight from Oxford Biomaterials, recognized that it's not so much what a spider spins as how it spins it that leads to the silk's remarkable properties. "What the spider does that makes spider silk so incredibly strong but elastic is that it aligns the silk fibroin molecules very closely in the fiber," Skaer states.
While spider silk possesses the mechanical and tensile properties suitable for orthopedic applications, scientists cannot rely on spiders as a source of the raw material because the arthropods do not produce enough of it for human commercial purposes. In contrast, while other silk-producing organisms such as the mulberry silk moth and the mulberry silkworm have been cultivated for millennia, they introduce weak points into the fiber so that they can break out of their cocoons at the end of the pupation period. Because of this intrinsic weakness, raw silkworm silk is too weak to be used for bone or cartilage replacement applications.
To overcome this dilemma, Orthox exploits the respective properties of both spider and silkworm silk. The company begins with commercially sourced silkworm silk, breaks it down into its individual molecules using proprietary patented processes, and builds it back up into tissue scaffolds—essentially producing very porous silk protein sponges. In doing so, the company removes all of the weak points in the fiber. "We put it back together again as a spider spins its silk, and that allows us to capture the great strength and resilience that you see in spider silk," Skaer explains. "But we use a commercially viable source of raw material—silkworm silk."
The company's strategy works because spider silk proteins (spidroins) and silkworm silk proteins (fibroins) have the same basic molecular structure. Pleated-sheet proteins, they have a strong tertiary protein structure that is homologous to the human protein fibronectin. "As their names suggest, all of these substances are fiber-forming proteins, and they have very good structural and mechanical properties. That's what spiders and silkworms use to spin webs or cocoons, and that's what you find in the extracellular matrix of cartilage and bone tissue," Skaer says.
Cartilage replacement and repair is an enormous, unsolved clinical problem—especially in light of extended longevity, human's desire to remain active in later life, and the growing problem of obesity, all of which can burden joints and lead to osteoarthritis. Furthermore, cartilage tissue heals only very slowly or not at all in people over the age of 30. Cartilage damage is particularly deleterious to the knee. Studies indicate that in the next two decades, knee replacements in the United States will increase by 525% to 3.4 million a year.
Confronting the unsolved challenge of developing cartilage-replacement material for an aging and active population, Orthox has developed a tissue scaffold known as FibroFix—a strong, resilient, but porous material resembling fibronectin that the company hopes will one day be used to replace damaged meniscal cartilage—the knee's shock absorber. "FibroFix can perform the same shock-absorbing function as the meniscal cartilage," Skaer remarks, "and we've demonstrated in the lab that it can do that for more than 3-million knee-load cycles, which is at least a couple of years of wear in your knee." However, in the longer term, the company hopes that the material's porous structure and its similarity to the proteins found in cartilage will enable cartilage tissue to grow into it, eventually effecting a regenerative repair. "So it's a short- to medium-term functional replacement, and it's a long-term regenerative solution," Skaer says.
Seeking Regenerative Solutions
While biomimetics may be here to stay, progress sometimes seems to advance at a snail's pace. "Spiders have been learning how to spin their silks for 400 million years," Skaer comments, "and we can't expect to replicate what they do in four or even 40 years." Nevertheless, he believes that nature has come up with many solutions from which scientists can draw inspiration, bearing in mind that these solutions were adapted through evolutionary processes over hundreds of millions of years and that emulating them will be an extended process.
Messersmith believes that science should be working toward developing regenerative solutions for many diseases. "One area that I think is going to take some time to develop but will be a major force in the future is the idea of incorporating self-healing capabilities into materials that are used in medical devices. And by that I mean materials that have the inherent ability to heal damage that's induced by forces such as mechanical loading." For example, cracks that form in materials can heal during periods of low loads. While this is a common feature of biological materials in nature, the scientific community has not yet been able to capitalize on this concept and transform it into materials that are used in medical devices. "But I think it's a matter of time and people," Messersmith adds. "Particularly in academic research, the idea of designing materials with this capability is gaining a lot of momentum. I think it's a matter of time before we see industry make some headway in this field."