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Rattan-Based Bone Implant Mimics Nature

In the search for new biomaterials, scientists are learning to emulate the attributes of animal and plant organisms

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Scientists have studied a range of materials as potential candidates for bone implants, including bone from cadavers, artificial wood-based bone, metals, and ceramics. But all of these materials have their drawbacks. Bones from cadavers, for example, are scarce, expensive to store, and potential transmitters of disease. And while metals and structural ceramics such as alumina and zirconia have been used for years as bone-replacement materials because they can replace long portions of bone that are subjected to mechanical loads, they are bioinert, preventing them from playing a biological role in the process of bone regeneration and healing. Moreover, because these materials exhibit higher mechanical strength and rigidity than natural bone when subjected to loads, they often result in complications requiring follow-up surgeries--especially in younger patients.

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A cross-section image of rattan illustrates the material's aligned channels, which are wide enough to allow cell attachment and colonization.
Dedicated to discovering more-effective bone-replacement materials, a group of researchers at the Institute of Science and Technology for Ceramics (ISTEC; Faenza, Italy; www.istec.cnr.it) has turned to the flora of the tropics. Through their efforts, the scientists have learned how to turn rattan--the name for approximately 600 species of palms--into a material that is almost identical to human bone tissue.

"Among the different natural structures we have investigated so far, rattan wood exhibits a structure very close to that of natural bone tissue," explains Anna Tampieri, group leader for bioceramics and biohybrid composites at ISTEC. Rattan's structure is strongly anisotropic, containing aligned channels running throughout the wood that are wide enough to allow cell attachment and colonization inside the 3-D structure. This morphology, she adds, also promotes angiogenesis, a crucial feature for enabling the exchange of nutrients and waste products and for integrating the rattan scaffold into the surrounding bone.

"Apart from its chemical composition, bone's remarkable properties of lightness, resistance, and capacity for self-regeneration are due mostly to its hierarchically organized structure, which is not possible to reproduce using current manufacturing technology," Tampieri says. "In looking to rattan, we took inspiration from nature, which exhibits innumerable astonishing structural forms--wood, plants, shells, corals, carapaces, etc.--that have been optimized through millions of years of evolution."

The current synthetic material of choice for bone substitution is hydroxyapatite, a calcium-phosphate material with a composition resembling the inorganic part of bone. However, while cells recognize hydroxyapatite as a source of calcium and phosphorus and can incorporate it to form new bone tissue that completely replaces the bone implant, the material has poor mechanical strength and cannot be used as a substitute for load-bearing bones. Despite this limitation, a biomimetic material such as hydroxyapatite, organized in a hierarchical manner similar to that of bone, is a fascinating and promising substitute for all kinds of bone tissue in the future, according to Tampieri.

That's where rattan comes in. To acquire optimized hydroxyapatite suitable for use in bone implants, the scientists process native rattan in five steps. First, the wood is cut into pieces with the desired shapes and sizes, after which it is dried and subjected to a thermal oxygen-free treatment at 1000° to 1100°C. This process eliminates organic components, leaving a skeleton composed of pure carbon, and reproduces the complex microstructure of wood. Then, this carbon template reacts with gaseous calcium in an oxygen-free, high-temperature environment, forming calcium carbide. This material is then converted into calcium oxide by heating it at 1000°C in a combination of air and calcium carbonate in flux or under pressurized carbon dioxide. Finally, the calcium carbonate pieces are transformed into hydroxyapatite, reproducing the mineral part of bone. This last step is performed by soaking the pieces in a phosphate solution under temperature-controlled conditions so that the end material contains carbonate ions and will likely behave in vivo very much like natural bone.

"Once transformed into hydroxyapatite, rattan is perfectly biocompatible, and tests have demonstrated this," Tampieri says. "Since the chemical composition of the material obtained is the same as that of natural bone, biocompatibility is not a major concern." In fact, the capacity of the hydroxyapatite scaffold to integrate into existing bone promotes osteogenesis and angiogenesis while lending biomechanical properties to the fused structure.

All of these steps for processing rattan into hydroxyapatite, according to Tampieri, are carefully optimized to maintain the original microstructure of the wood. They are also affordable, requiring conventional furnaces, an autoclave, and basic chemical laboratory equipment. While the entire procedure for completely transforming native rattan into implant-capable hydroxyapatite takes approximately two to three weeks, the Italian team is investigating how to optimize and reduce the processing time.

It is likely that other woods can be processed to function as bone-replacement material, Tampieri comments. "The criteria for the proper selection of wood are a strongly oriented microstructure, the presence of channels wide enough for hosting cells, and the mechanical properties of the wood itself, which are reflected in the properties of the final bone scaffold." The research team is also trying to transform red oak and sipo into biomorphic hydroxyapatite, since their more-compact structure could be suitable for reproducing the dense cortical part of bone, which surrounds the spongy, more-porous and biologically active part. In addition, other types of tropical woods are being studied.

Tampieri predicts that the rattan-based scaffold will be tested on human patients within five years. In the meantime, her team is focused on implanting the material into sheep with critical metatarsus defects, the purpose of which is to assess the capacity of wood-derived materials to integrate into existing bone after a few months and to evaluate the scaffolds' mechanical resistance. Validating the use of wood-derived materials as bone scaffolds, Tampieri hopes, will be the starting point for future activity devoted to strengthening reabsorbable implants made from wood-derived hydroxyapatite.
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