When a bone is injured, there are very few available treatments. One option is to repair the injury with bone harvested from another part of the body, which limits the amount that can be repaired. Another is to use bone tissue from cadavers, which stands a strong chance of being rejected by the recipient’s body. These limitations have paved the way for the growing field of bone tissue engineering. Research in this field has produced an array of potential materials for repairing or replacing damaged bone, one of which is being developed by Dr. Balaji Sitharaman in Stony Brook’s biomedical engineering department. In a study recently published in the scientific journal Acta Biomaterialia, Dr. Sitharaman reports his lab’s recent advancement in the field.
Many of the materials currently being developed involve the same basic construct. A flexible, biodegradable polymer forms the basis of the material and when combined with tiny structures called nanotubes, the polymer is transformed into a strong bone-like material. Gaurav Lalwani, a fourth year graduate student in the Sitharaman lab and an author of the study, compares this combination of materials to concrete. Alone, cement can only support a limited amount of weight, but when gravel is mixed in, forming concrete, its strength increases enormously. The nanotubes in this engineered material act as the gravel. Alone, the polymer base can only support a limited amount of force. However, the nanotubes, though only a fraction of the width of a human hair, are many times stronger than steel and support the force that the polymer itself could not. Previous versions of this material have typically been made with carbon-based nanotubes and these feature inherent limitations. When mixed with the polymer base, these nanotubes form aggregates, or “clumps.” Dr. Sitharaman explains that when the nanotubes are not uniformly distributed throughout the polymer, the resulting material will not uniformly support force, creating a material not strong enough to withstand extensive compression or bending–forces typically applied to bone. Additionally, when the nanotubes aggregate together, they make fewer connections with the surrounding polymer, and have a tendency to slip against each other when applied with force. These materials are adequate for bones like those in the arms that do not need to withstand constant force, but for bones like those in the leg, they are insufficient. As Lalwani states, “They fail to take the load.” To resolve these problems, Dr. Sitharaman and his students created a material using different types of nanotubes. Rather than the standard carbon-based tubes, they used an inorganic material: tungsten disulfide. A small amount of these inorganic nanotubes, which to the human eye have the appearance of a fine, grey powder, combined with the polymer base, which has the consistency of honey, form a strong, solid material that can be formed into a variety of shapes. The material can then be compressed and bent to test its strength. The group found that not only do the inorganic nanotubes distribute themselves more uniformly throughout the polymer, they also form more connections to the polymer itself, compared to their carbon counterparts, and create a stronger, more flexible material: a material much more suitable for load-bearing bones, like those in the leg.
Bone tissue engineered in labs is not meant to be a permanent replacement for damaged bone. Rather, the tissue is meant to provide a structure which bone can grow into and develop. While the natural bone grows and matures, the engineered tissue degrades, eventually leaving behind fully developed, natural bone. For this to occur, the engineered tissue must be porous in order to allow human cells to infiltrate and mature into natural bone tissue. According to Dr. Sitharaman, this is his lab’s next step. They will introduce pores into the inorganic nanotube construct and test the material’s strength as they optimize the balance between pore density and tissue durability. Lalwani explains that salt will be mixed in with the polymer and nanotubes. The resulting solid will be placed in water, dissolving the salt and leaving behind pores: a simple, elegant method for advancing an important biomedical development and moving this work one step closer from the lab to the clinic.