BUILDING BONES: A LOOK INTO LATTICE CONSTRUCTION

Edward PaceyNathan Kresman

Constructing the Scaffold

Case Studies: Using Synthesized Bone Segments

Human Bone: An OverviewEthical Issues: Nanotechnology and Bone Tissue

Engineering

Future Applications of Bone Tissue Engineering

To understand the structure of synthesized bone, the structure of human bone must be explained first. Human bone is constructed in a lattice framework. This framework is a composite of tough, yet flexible collagen fibers reinforced with calcium phosphate nanocrystals. This structure supplies the tissue with greater stiffness while simultaneously providing high resistance to fracture and various external forces. This lattice framework in arranged in a highly organized hierarchal structure. Collagen proteins form fibrils about 500 nanometers in diameter, which then form into collagen fibers. These fibers are primarily collagen and hydroxyapatite. The fibers form lamellas, which in turn combine to form osteons, constituting the macrostructure of the bone. The image below visually represents this hierarchal structure of bone. The cortical bone, or hard outer surface, protects the inner, spongy tissue known as trebeculae. This tissue is primarily composed of collagen fibers, and ultimately gives bone its elastic property. The trebeculae, as well as other collagen fibers, are stacked in parallel to form layers and assembled at a 45 degree angle to the cortical bone. This allows the formation of pores approximately 300 micrometers in diameter to form along the entire surface of the bone. These pores will promote optimal vascularization and blood flow, allowing cells to grow throughout the bone.

Although synthesizing bone segments is a relatively new innovation, trials and experiments have been undertaken by numerous professionals in the field of bone tissue engineering. In the field of medicine, a major problem that confronts many doctors and engineers is repairing large segmental bone defects due to trauma, inflammation, and tumor surgery. Methods such as bone grafts and bone transplants have been used for years. However, these methods often have consequences and typically cause more issues for the patient after the process. Due to these issues, engineers began to research the use of synthesized bones in animal models. Kon first evaluated the potential of bone marrow stromal cells to repair defective bones in vivo in sheep models. He observed bone formation in seeded and non-seeded structures using a 100% hydroxyapatite scaffold. The seeded structure, however, showed better formation within and around the entire scaffold, and quality pore formation for later growth. Petite also experimented with BMSCs, but used a coral-based scaffold as an alternative frame. He concluded that the distribution of cells over the scaffold affects the bone-forming ability of the transplant. He also found that coral-based hydroxyapatite scaffold formed more bone in comparison to a natural coral scaffold. 70% of models healed using the former, while only 14% healed using the latter. Although very limited, testing has been done on human subjects as well. Quarto and colleagues were the first to report the repair of massive bone defects in humans. Hydroxyapatite scaffolds were custom made to match each defect, and seeded with osteoinductive cells. No negative or adverse side effects were reported in any of the patients. Fusion of the segment occurred five to seven months after, and all patients regained total function of their limbs six to twelve months after the surgery. Further, the National Institute of Advanced Industrial Science and Technology succeeded in treating three patients with benign bone tumors using synthesized bone segments. The engineers extracted BMSCs and expanded them in vitro, seeding them with the cells and then placing each segment into each patient’s bone cavity. Immediate healing was noticed, and no adverse effects were reported by the three patients.

Forming the Internal Components Like many medical innovations and technologies, synthesizing bone segments has a number of ethical issues confronting it. Cell donation, for example, can present an issue to the public. When people go for testing and subsequently get bone, blood, and tissue samples extracted, these samples must be kept private and not tampered or given out to anyone. If tampered with, further surgeries done to that patient could go awry due to the incorrect information in the patient’s samples. Further, because these samples are of blood and tissue, that person’s genetic code lies within them. If these samples were to be given to anyone without the consent of the patient, serious legal consequence could arise for the company involved. Another issue in bone tissue engineering is the use of human embryonic stem cells. Despite years of research, hESCs are one of the most controversial topics in the medical field. They are highly valued for their versatility due to their pluripotent attribute and their ease of extraction. However, because they can only be extracted from embryos donated by humans, ultimately being destroyed, there is much debate as to when an embryo biologically becomes a child. Many disagree with the use of these cells in any way, as it interferes with many religions and beliefs. However, engineers must take action to demonstrate to the public that the positive outcomes and purposeful applications of these cells will outweigh any negative consequences that result from them. For more ethical issues in bone tissue engineering, refer to the table below.

Quality of Life: Sustaining the Future of Medicine

When creating technology and coming up with new innovations, engineers must keep in mind how this technology will affect the current generations and future ones alike. With that being said, the use of nanotechnology must remain sustainable so that future generations can benefit from it just as we have. However, despite the many innovations that have resulted from nanotechnology, society may already have to reconsider this technology until further research is completed. The biomaterials used in nanotechnology, particularly in the bone tissue engineering field, include silver, aluminum, calcium, silicon, ceramics, and a number of other materials. Externally, these materials do not cause harm to humans. However, past observations show that if one of these materials were to isolated within the human body, the consequences could be severe. A biological reaction could potentially occur, and the severity depends upon the toxicity and length of exposure to the substance. If a ceramic scaffold were to break within the arm of a human, the body will likely not accept these foreign particles, and use any means necessary to get rid of it. This could result in a severe reaction or even death. Therefore, for nanotechnology and the fields associated with it to advance, laws and regulations must be passed to allow such materials to be used within the body. Action must be taken to build a universally accepted body of knowledge that addresses the benefits and risks of commercialized nanomaterials to avoid future consequences

The internal components of synthesized bone include the nanofibers and the medium surrounding them. These nanofibers are primarily comprised of collagen compounds reinforced with apatite crystals. They are made through a process known as electrospinning, which is charging a suspended droplet of polymer melt and spinning it to the desired diameter. After they are integrated within the scaffold, a process known as phase separation creates a 3D porous network within the nanofibers to allow cells to grow in and around them. Nanofibers pervade the inner structure of the scaffold because they provide substantial surface area for cell attachment, and allow cells to grow in the correct orientation with optimal growth. The cumulative substance that comprises the medium of the scaffold as a whole is known as the extracellular matrix, or ECM. It provides cell support and directs cell behavior through cell-ECM interactions. In order to replicate the ECM, a non-mineralized organic compound and a mineralized inorganic compound, such as collagen and apatite mineralites, must be used. In essence, the ECM is responsible for maintaining a microenvironment that will hold all internal components of the synthesized bone segment. This includes the nanofibers, hydrogel, osteoinductive cells, or any other biomaterial used to create the bone. The ECM is necessary for regeneration of tissues, and will ultimately allow cells to permeate the scaffold proficiently.

The biocompatible scaffold acts as the frame for the synthetic bone. There are many materials that can be used to make the scaffold, including hydroxyapatite, ceramic, coral, aluminum, and others. The scaffold is made through the process of 3D printing. Essentially, the scaffold is made by hardening the given material using the laser to make the initial frame. The unhardened material is then removed using a solution that removes the non-hardened material into a liquid. This in turn creates an extremely lightweight structure, yet an extremely strong structure. Patterns that comprise the structure can also vary. Triangles, hexagons, and diamonds are just a few examples that can be used to create a scaffold frame. Triangles are most commonly used, however, because they can bear the most weight of any geometric figure. Pore size is also important when creating a biocompatible scaffold. If the structure is less obtrusive to the surrounding tissue, the faster it will be populated with cells. Essentially, the objective is to have enough framework to support the structure but not impede the movement of the cells by having excess framework.

Applications in bone tissue engineering progress each day as research continues. Being in close association with nanotechnology, both fields bring many new innovations to the medical field. Nanomedicine, in particular, has shown great potential in recent years. Applications such as in vitro diagnostics and in vivo imaging have come to the forefront of the medical field. These applications could allow doctors to actively detect disease biomarkers as well as detect viruses and bacteria in early stages to prevent disease. Further, this technology could help detect tumors and sentinel lymph nodes in order to detect possible cancer cells. Engineers have taken full control of regenerative therapies, and the future looks promising. There is already a strong understanding and significant testing in the areas of skin, tissue, and organ regeneration. Once synthesizing bone segments is mastered, engineers plan to use this process to create entire bones for people lacking limbs. However, ethical issues and regulations associated with these applications must be handled foremost. Further, engineers and doctors alike have yet to grasp a solid understanding of the nervous system and regenerating nerves. Despite the overwhelming positive outlook by professionals, it could be quite some time before full limb regeneration is a possibility. Until then, it remains only an idea in the aspiring minds of today’s engineers.