A Texas A&M University researcher has created a shape-shifting material that can mold itself and fill unsightly gaps in bone, while promoting bone growth, which she says could be used to effectively treat defects in the facial region.
The research by Texas A&M Department of Biomedical Engineering associate professor Melissa Grunlan Ph.D.,M.S., B.S. is detailed in the scientific journal Acta Biomaterialia. Working with colleagues at Texas A&M and Rensselaer Polytechnic Institute, Dr. Grunlan has created a polymer foam that is malleable after treating with warm saline, allowing it to precisely fill a bone defect before hardening into a porous, sponge-like scaffold that promotes new bone formation.
The Acta Biomaterialia article, entitled “A bioactive “self-fitting” shape memory polymer scaffold with potential to treat cranio-maxillo facial bone defects” (DOI: 10.1016/j.actbio.2014.07.020 ), is coauthored by Dr. Grunlan with Dawei Zhang of the Department of Materials Science and Engineering, Texas A&M University; Olivia J. George and Keri M. Petersen of the Department of Biomedical Engineering, TAMU; and Andrea C. Jimenez-Vergara and Mariah S. Hahn of the Department of Biomedical Engineering at the Rensselaer Polytechnic Institute in Troy, New York.
The researchers note that while tissue engineering is a promising alternative for treating critical-sized cranio-maxillofacial bone defects, improvements in scaffold design are needed — in particular, scaffolds that can precisely match the irregular boundaries of bone defects as well as exhibit an interconnected pore morphology and bioactivity would enhance tissue regeneration.
The coauthors explain that in this study, a shape memory polymer (SMP) scaffold was developed exhibiting an open porous structure and the capacity to conformally self-fit into irregular defects. The SMP scaffold was prepared via photocrosslinking of poly(-caprolactone) (PCL) diacrylate using a SCPL method, which included a fused salt template. A bioactive polydopamine coating was applied to coat the pore walls. Following exposure to warm saline at T > Ttrans (Ttrans = Tm of PCL), the scaffold became malleable and could be pressed into an irregular model defect. Cooling caused the scaffold to lock in its temporary shape within the defect.
The polydopamine coating did not alter the physical properties of the scaffold. However, polydopamine-coated scaffolds exhibited superior bioactivity (i.e. formation of hydroxyapatite in vitro), osteoblast adhesion, proliferation, osteogenic gene expression and extracellular matrix deposition.
The research team envisions the material as a treatment for cranio-maxillofacial bone defects gaps in bone occurring in the head, face, or jaw areas. These defects, which can dramatically alter a person’s appearance, can be caused by injuries, birth defects such as cleft palates, or surgical procedures such as the removal of tumors, Dr. Grunlan says.
In order to repair these defects, the polymer foam developed by Dr. Grunlan and her team acts as a scaffold, which is a temporary structure that supports the damaged area while promoting healing by allowing bone cells to migrate into the area and repair the damaged tissue. Ultimately, the scaffold dissolves, leaving behind new bone tissue, she explains in a release.
“Try as hard as we do to create artificial materials to replace damaged or diseased tissues, it is nearly impossible to match the properties of native, healthy tissue and so the whole idea behind tissue engineering is that if we can restore native-like, healthy tissue, that will be better than any artificial replacement,” Dr. Grunlan observes.
“A problem,” she adds, “is directing that process in these areas where there is a critical bone defect. In these types of instances where large gaps exist the body doesn’t have the ability to heal the defect with new bone tissue growth; we have to help it along, and that is what our material is designed to do.
Dr. Grunlan is associate professor and director of undergraduate programs in the Department of Biomedical Engineering at Texas A&M University. Her laboratory focuses on developing new polymeric biomaterials for medical devices and regenerative therapies (i.e. tissue engineering).
Hybrid systems based on combining inorganic and organic polymers are produced as coatings, hydrogels, elastomers and porous foams. Chemical, surface, mechanical and thermal properties are evaluated with a variety of techniques.
Current projects are focused on developing self-cleaning membranes for implanted biosensors, clot-resistant coatings for blood-contacting devices and the subject of the Acta Biomaterialia study — scaffolds for bone repair and for the regeneration of osteochondral interfaces.
The team at the Grunlan Research Group lab designs new polymeric biomaterials to improve performance of medical devices as well as regenerative therapies. A distinction of the Grunlan lab is inorganic-organic hybrid materials prepared by combining inorganic silicon-containing polymers with organic polymers.
Biomaterials are currently defined as a material intended to interface with biological systems to evaluate, treat, augment, or replace any tissue, organ, or function of the body. Today, polymeric biomaterials constitute ~70 % of implantable medical devices. Polymers are utilized for a range of devices, including those for ophthalmic, cardiovascular, orthopedic, therapeutic and regenerative applications.
Prof. Grunlan is also head of the Silicon-Containing Polymeric Biomaterials Group (http://biomed.tamu.edu/biomaterials). A principal distinction of this research is the development of inorganic silicon-containing polymeric materials and their combination with organic polymers to obtain inorganic-organic hybrid materials with unique properties. Several specific research areas include: clot-resistant coatings for blood-contacting devices, self-cleaning membranes for implanted biosensors, and the scaffolds for tissue engineering and shape memory polymers (SMPs) for bone healing.
Key to Dr. Grunlan’s novel material is its malleability after brief exposure to warm saline (140 degrees Fahrenheit), a property that enables surgeons to easily mold the material to fill irregularly shaped gaps in bone. Once a defect is filled, the material cools to body temperature and resumes its stiff texture, locking itself in place, she explains.
“This self-fitting aspect of the material gives it a significant edge over autografting, the most common treatment for these types of bone defects,” Dr. Grunlan notes. “Autografting involves harvesting bone from elsewhere in the body, such as the hip, and then arduously shaping it to fit the bone defect. In addition to its obvious limited availability, the bone harvested through autografting is very rigid, making it difficult to shape and resulting in a lack of contact between the graft and the surrounding tissue, she continues. “When this occurs, complications can arise. For example, a graft can inadvertently dissolve through a process known as graft resorption, leaving behind the defect.”
“Another therapy involves filling the defect with bone putty, but that material can be brittle once it hardens, and it lacks the pores necessary for bone cells to move into the area and repair the tissue,” she continues.
By tweaking the polymer scaffold through a chemical process that bonds individual molecular chains, Dr. Grunlan and her team overcame that issue and produced a sponge-like material with interconnected pores. They also coated the material with a bioactive substance that helps lock it into place by inducing formation of a mineral that is found in bone, she adds. The coating, Dr. Grunlan explains, helps osteoblasts in the cells that produce bone to adhere and spread throughout the polymer scaffold. “Think of it as a sort of boost to the materials healing properties.”
Thus far, the researchers report that results have been promising. After only three days, the coated material had grown about five times more osteoblasts than uncoated versions of the same material, Dr. Grunlan says. In addition, the osteoblasts present within the scaffold produced more of the proteins critical for new bone formation. The team plans to continue studying the materials ability to heal cranio-maxillofacial bone defects by moving testing into preclinical and clinical studies.
Texas A&M UNiversity
Texas A&M UNiversity