FRG: Mechanically- and Biologically-Active Nickel-Titanium Foamas Biomimetic Material for Skeletal Repair

Northwestern University, Evanston IL

Investigators

Abstract

One of the great challenges in bioengineering is the effective repair of the human skeletal system. At the same time, bone is a very rich substrate to learn about structure-property relations in materials. Despite decades of research and the importance for human health and quality of life, ideal systems for skeletal repair have not yet been achieved. The ability to synthesize novel biomimetic micro- and nanostructured materials will enable the development of materials designed to trigger bone repair and perhaps to achieve the remarkable properties of bone in artificial systems. Porous metal coatings that allow bone ingrowth for device fixation have been developed, however success is limited by debonding of the coatings, as well as by the biological unpredictability of bone growth. This project explores the alternative approach of using fully porous metallic foam for the implant device, which both provides a porous structure ideal for bony ingrowth and lowers implant stiffness, thereby reducing stress-shielding problems seen with solid metallic implants. This approach combines with the modification of internal surfaces in titanium or titanium alloy foams with self-assembling polymers designed to be bioactive and this way recruit the appropriate cells to grow bone within the pore volume. The supramolecular polymers are also used in this project to encapsulate bone cells during their self-assembly in the interior of metallic foams. Analytical modeling of the porous Ti is used in the project to provide good estimates of the elastic moduli, and using finite element analyses stresses are predicted as bone fills the pores. This Focused Research Group project is being co-funded by the Polymers and Metals Programs in the Division of Materials Research and the Mechanics and Structures of Materials Program in the Division of Civil and Mechanical Systems. This interdisciplinary project aims to develop superelastic nickel-titanium metallic foams with moduli comparable to bone, which contain bioactive materials on their internal surfaces that invite rapid bone growth. These superelastic alloys may be useful for stimulating bone cells mechanically given their extensive recoverable deformation and will enable a novel insertion technique. The proposed work integrates the capabilities of three groups in metallurgical synthesis (Dunand), self-assembly (Stupp), and biomechanics (Brinson) to develop a mechanically- and biologically-active osteomimetic material with novel functions for optimized skeletal repair. Overlapping research and collaborative interactions will be pursued using the successful framework that was developed in a previous program on Ti foams. Porous NiTi samples created in the Dunand laboratory will be transformed into bioactive materials by self-assembly of supramolecular polymer coatings from the Stupp laboratory. These bioactive materials will be investigated for biological activity via bone ingrowth studies (Stupp) and linked to mechanical properties via cyclic in situ loading (Brinson). Feedback on the bioactivity (Stupp) as a function of loading, pore morphology and local properties from modeling (Brinson) will in turn suggest improvements in foam processing (Dunand). As a research area, skeletal repair has broad impact because it is not only critically important to human health, human productivity, and quality of life, but it is at the same time highly interdisciplinary, and can therefore have great impact on graduate and undergraduate education. The approach to the subject in this proposed program offers a great opportunity for interdisciplinary training of students, integrating advanced metallurgy, organic chemistry, cell biology and mechanics to investigate a complex problem. The project's interdisciplinary nature is also a good platform to generate new ideas on biomimetic design of microstructure and nanostructure in materials using a mechanically adaptable, self-healing material as the model.