May 5 2009
The National Institute of Standards and Technology (NIST) last week issued its first reference materials to support the new and growing field of tissue engineering for medicine. The new NIST materials are samples of a typical tissue scaffold material that have been measured and documented by NIST for three different degrees of porosity.
X-ray microcomputed tomograph image of the new NIST reference materials for tissue engineering scaffolds, (l to r) RM 8395, RM 8396 and RM 8397. Credit: NIST
Three-dimensional tissue scaffolds, under development for some years, are biodegradable materials that are meant to be implanted in the body to provide a structurally sound framework for the patient’s cells to implant and grow, in time repairing damaged tissue. The scaffolds are meant to be absorbed gradually by the body and replaced by normal tissue. Today they are used most commonly to help repair damaged bone, but other applications being studied.
In addition to biocompatibility and biodegradability, successful 3-D tissue scaffolds have a number of physical requirements. Porosity or pore size is one key factor. The pores in the scaffold must be large enough to permit cells to infuse the structure and receive nutrients, but healthy cell growth also depends on the cell’s immediate surroundings. If the pores are too large or spaced too far apart, cells will be unable to build the proper connections.
The three new NIST reference materials are disks approximately 20 millimeters across and 5 millimeters high formed of crisscrossed layers polyester struts approximately 200 micrometers in diameter. Varying the spacing of the struts in each layer resulted in three different average porosities for the disks: 47 percent (average strut spacing of 200 micrometers), 60 percent (300 micrometers), and 69 percent (450 micrometers). These span the common range of pore sizes typically required for tissue engineering applications.
The biodegradable polymer, polycaprolactone, originally was used for sutures, and was chosen for being relatively strong and stable when not exposed to water or sunlight. The material has been approved by the Food and Drug Administration for use in tissue engineering implants, but the NIST reference materials are not meant for use in the body.
The release of these reference materials culminates a multi-year effort involving input from the FDA, the National Institutes of Health and ASTM Working Group WK6507 “Reference Scaffolds for Tissue Engineering.”