By John P. Roche
Carbon has four valence electrons, allowing it to form four bonds with itself, which makes it uniquely versatile as a material. Carbon can bind to itself to form substances that include diamond, which is a solid three-dimensional crystal; graphene, which is a two-dimensional sheet of pure carbon; and carbon nanotubes, which are sheets of carbon folded in upon themselves to form cylinders. Each of these forms of carbon has myriad practical uses. Yet another form of carbon with important practical applications is glassy carbon, also called vitreous carbon. One form of glassy carbon is a reticulated—that is, porous—foam known as reticulated vitreous carbon (RVC) foam. RVC foams are attractive for biomedical applications because they are strong, and because their high resistance to high temperatures allows them to be sterilized. RVC foams have potential applications to the treatment of bone injuries.
Injuries to bones affect millions in the U.S. each year. Bone grafts are used to restore seriously damaged bone, or to grow bone around an implant such as a knee replacement. Material for grafts can come from sections of bone from other individuals, sections of bone from a patient’s own body, or man-made materials. Grafts using bone material present several serious problems, including the risk of infection, a potential loss of function, and limitations in the amount of tissue available for use in grafts. In order to solve these issues, investigators have been studying grafts using biomaterial composites. One such biomaterial composite is RVC foam.
RVC foams are ideal for grafting because being porous, they have a large surface area, and thus provide an excellent scaffold to aid the process of tissue regeneration. However, work testing RVC foams have found issues with strength—and given the considerable stresses placed on bones in the human body, strength is an important prerequisite for an effective biomaterial graft. Pure RVC foams are not strong enough to withstand the normal functional stresses occurring during the period in which bone regeneration and repair take place. Investigators have tried testing non-degradable polymer polytetrafluorethylene (PTFE) for use as a coating to strengthen RVC foams, but PTFE-coated foams did not add sufficient strength, and they also can cause negative reactions wherein the body responds to them as a foreign material.
To test an alternative method of adding strength to RVC foams, Douglas Rodriguez of Texas A&M University, Viviana Guiza-Arguello and Mariah Hahn of Rensselaer Polytechnic Institute, and their colleagues infiltrated an RVC foam with a filler that can be absorbed by the body over time as the bone regrows. They tested two types of filler, a poly(D,L-lactide-co-glycolide)—that is, PLGA (shown below), and PLGA reinforced with hydroxyapatite.
PLGA is biocompatible, it has been approved by the U.S. Food and Drug Administration, and it offers the advantage of being biodegradable, which metal and ceramic implants are not. In their recent study, published in the journal Carbon, Rodriguez et al. tested a RVC composite foam infiltrated with PLGA to examine its potential as a superior biomaterial for bone grafts. They tested three characteristics of the PLGA-infiltrated RVC foam to investigate its promise as a grafting material: (1) the stress response of the composite material under compression loading; (2) the rate of biodegradation of the filler; and (3) the degree of cell adhesion and cell spreading possible in the foam.
To test the affect of infiltration of PLGA on RVC, they compared control RVC samples not infused with PLGA with RVC samples infused with PLGA. The figure below shows a block of RVC foam infiltrated with PLGA (the brown block with dark spots) and an overlay illustrating the structure of the RVC foam within the infiltrated material. [Image courtesy of Mariah Hahn.]
The study’s results were encouraging. In their test of compression loading, Rodriguez et al. found that the samples of RVC infiltrated with PLGA was fifty times more resistant to failure due to compression stress than RVC foam without PLGA. In their test of biodegradation rate, they found that the PLGA in the PLGA-infiltrated RVC foam degraded over time, as desired, and that the degradation was slow enough to maintain strength of the composite during degradation. In their test of cell adhesion and cell spreading, they found that the PLGA-infiltrated RVC foam did not interfere with adhesion of cells and spreading of cells into the foam, also as desired if this composite is to be developed as bone graft material.
The figure below shows a cross section of a sample of a PLGA-infiltrated RVC foam. This cross section illustrates void areas not filled with PLGA. In their study, Rodriguez et al. estimated void content in their PLGA-infiltrated RVC foam samples and found that the void content was only 5–10 % of their volume. [This figure is reprinted with permission from Elsevier: Copyright © 2015 Elsevier Ltd. All rights reserved. This is a panel from a figure that originally appeared in the journal Carbon.]
This study showed that PLGA-infiltrated RVC foam displays robust strength under compression forces, and that it permits cell adhesion and cell spreading, facilitating bone regeneration following grafting. Rodriguez, Guiza-Arguello, Hahn, and colleagues note that future research should also investigate the degree to which the material provides strength in the face of other forces, such as twisting (torsion) stress and shearing stress. In an interview, Dr. Hahn commented, "We anticipate that initial in vivo studies of an optimized infiltrated carbon foam composite will be completed within the next 1-2 years." These in vivo studies in living organisms will shed further light on the promise of this exciting application of carbon.
To read the article in the journal Carbon, click here.