Before introduction, however, stringent testing must be performed to verify their strength and durability over time. For this fatigue testing, implant developers often rely on infused-epoxy composite bones from Pacific Research Laboratories Inc., Vashon Island, Wash.
The composite bones are molded from a two-part epoxy system reinforced with chopped glass fibers. Physical properties (85 Shore D hardness, an elongation of 2%, and tensile and compressive strengths of 13 and 17.4 kpsi, respectively) are comparable to that of human bones and an added benefit is consistent performance. Cadaver bones previously used for testing implants varied in strength due to donor age and health, which hampered researchers' efforts to accurately compare results from one test to another.
According to Pacific Research Laboratories President Forrest Miller, the company's first attempt at building artificial bones for testing implants were made using unidirectional fiberglass cloth and epoxy resins. But these models didn't fracture like real bones. To get more realistic and consistent stress/strain properties the company switched to a two-part RenInfusion 8601 epoxy system from Huntsman Advanced Materials, East Lansing, Mich.
Each modeling project starts with a bone from a medical cadaver. Through a series of process steps, Pacific Research creates an epoxy tool for molding the composite bones. The RenCast 2000 epoxy used to cast the tooling, also from Huntsman, exhibits low shrinkage (0.001 in./in.), so the composite bones mold to exacting tolerances. The tooling epoxy also doesn't retain excessive heat so molded parts cool quickly. And the tooling is durable enough to mold more parts per day than rigid polyurethane tooling.
The tooling can also be machined and patched when alterations are required. And with a heat-deflection temperature of 446°F (230°C), the tool withstands the 200°F (93°C) temperatures and pressures used in producing the glass-filled composite bones.
To make the composite bone, technicians typically work with several molds at a time. While one part cures, they clean, release, and prepare a second mold for part injection. RenInfusion epoxy's extended gel time of more than 2 hr facilitates handling. After the resin and hardener are thoroughly mixed with fiberglass, the resulting slurrylike mixture is degassed and poured into a pressure pot. The mixed resin system is heated to 200°F and injected into the closed mold at 40 psi. Injection time is approximately 3 min/part; pressure is maintained until the resin begins to gel to minimize shrinkage. Bones cure for 30 min at room temperature before they are demolded, cleaned, and cores removed. "A key to producing the quality bones required for medical applications is maintaining tight tolerances on pressure and temperature in the pot and precise metering of the mixed resin system into the mold," Miller says.