corporate fiber reinforcements in a polymer matrix (e.g., carbon fiber/polyethylene matrix). These materials are convenient to mold and, by selection of reinforcing fiber quantity and length, the resultant properties may be tailored over a fairly broad range. This property range can be estimated, at least to a first approximation, by the methods developed here. Finally, further extension of this work should respond to the explicit limitation described above. Methods for three-dimensional analysis with nonlinear material behavior will yield more faithful simulations of real tissue behavior and will be applicable to soft tissue. Analytical development should be guided by experimental determination of actual orientation and orientation dispersions as well as the degree of interlamellar contiguity and individual fiber configurations. Basic properties of unidirectional bone-tissue microstructural elements should be measured, and procedures should be developed for the reliable experimental characterization of bone tissue samples as orthotropic (or anisotropic) lamellar materials. 8. References [1] Evans, F. G., Ed., Studies on the Anatomy and Function of Bone and Joints, pp. 92-112, 121-141 (Springer Verlag, New York, N.Y., 1966). [2] Buckles, R. G., Ed., Formation and Structure of Calcified Tissue, Advances in Bioengineering, (American Institute of Chemical Engineers Symposium, Series No. 114, 67, 1971). [3] Currey, J. D., Three analogies to explain the mechanical properties of bone, Biorheology 2, 1-10 (1964). [4] Dempster, W. T. and Liddicoat, R. T., Compact bone as a non-isotropic material, American J. Anatomy 91, 331 (1952). [5] Evans, F. G. and Vincentelli, R., Relation of collagen fiber orientation to some mechanical properties of human cortical bone, J. of Biomech. 2, 63 (1969). [6] Bonfield, W., Mechanism of deformation and fracture in bone, Composites 2, No. 3, 173–175 (June 1971). [7] Bonfield, W. and Clark, E. A., Elastic deformations of compact bone, J. Mat. Sci. 8, 1590 (1973). [8] Cooke, F. W., et al., The fracture mechanics of boneanother look at composite modeling, J. Biomedical Mat. Research Symposium: Materials and Design Considerations for the Attachment of Prostheses to the Musculoskeletal System (Interscience, New York, N.Y., 1973). [9] Currey, J. D., The relationship between the stiffness and the mineral content of bone, J. Biomech. 2, 477-480 (1969). [10] Lees, S. and Rollins, F. R., Jr., Anisotropy in hard dental tissues, J. Biomech. 5, 557 (1972). [11] Kenedi, R. M., Ed., Advances in Biomedical Engineering, pp. 162-168, 202–212 (Academic Press, New York, N.Y., 1971). [12] Cox, H. L., The elasticity and strength of paper and other fibrous materials, Brit. J. Appl. Phys. 3, 72-79 (1952). [13] Robinson, E. Y., On the Elastic Properties of Fiber Composite Laminates with Statistically Dispersed Orientations, (Proc. of the 28th Annual Technical Conference, Reinf. Plast. and Composites, Soc. of the Plastics Industries, New York, N.Y., 1973). [14] Giltrow, J. P. and Lancaster, J. K., Friction and wear of polymers reinforced with carbon fibers, Nature 214, 1106 (1967). [15] Musikant, S., Quartz and Graphite Filament Reinforced Composites for Orthopedic Surgical Application, J. Biomed. Mat. Research Symposium; Medical Application of Plastics (Interscience, New York. N.Y., 1972). [16] Sclippa, E., and Piekarski, K., Carbon fiber reinforced polyethylene for possible orthopedic uses, J. Biomed. Mat. Research 7,59 (1973). [17] Structural Design Guide for Advanced Composite Materials, Second Edition Prepared under AF Material Laboratory Contract No. F33615-69-C-1368 by Los Angeles Division of North American Rockwell Corp., Los Angeles, Ca., January, 1971. [18] Ashton, J. E., Halpin, J. C. and Petit, P. H., Primer on Composite materials (Technomic Publishing Co., Stamford, Conn., 1969). [19] Haut, C. H., and Little, R. W., A Constitutive Equation for Collagen Fibers, J. Biomech 5, 423 (1972). [20] Lukes, R. S. and Katz, J. L., Transformation of the Viscoelastic Functions of Calcified Tissues and Interfacial Biomaterials into a Common Representation (Fifth Annual Clemson Biomaterials Symposium, April, 1973). NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 415, Biomaterials, Proceedings of a Symposium held in conjunction with the Ninth Annual Meeting of the Association for the (Issued May 1975). A Simple In Vitro Test for Screening the Blood Compatibility of Materials H. Kambic, T. Komai, R. J. Kiraly, and Y. Nosé Department of Artificial Organs The Cleveland Clinic Foundation, 9500 Euclid Avenue, An in vitro blood compatibility test was developed to evaluate thromboresistant properties of materials. This method is called the closed-cell kinetic blood coagulation test. A closed cell system eliminates any air-blood interface. The blood is withdrawn directly from the animal into the cell, minimizing the exposure to foreign surfaces other than the one being studied and eliminating the use of anticoagulants through the process. The technique includes the simultaneous blood filling of eight cells with the test materials, and eight cells lined with a control material. As a control material we have selected silicone rubber, which has reasonably good thromboresistant properties, is widely accepted, and commercially available. The cells are opened at different predetermined times, and the clot formation is then measured by two complimentary methods: weighing the clot and colorimetry of the unclotted blood. The two methods correlate and can differentiate between red and white thrombus. The results are presented as clot formation curves versus time for the material under test and for Detailed analyses of the curves will offer a new approach to the understanding of the mechanism of 1. Introduction A large number of materials is available for use in intravascular devices. Since the processes occurring when materials are brought in contact with blood are not well known, a screening test is required in order to establish their material blood compatibility. The advantages of an in vitro screening test are obvious: low cost, independence of flow dynamics, and short time. However, questions have been raised as to their reliability as well as their correla tion with the actual behavior of the materials once implanted in vivo. A simple technique designed in our laboratory has been reported [5]. Further refinement has resulted in a more reliable and reproducible test [11]. This present report describes the results. obtained studying candidate materials with the technique. › Figures in brackets indicate the literature references at the end of the paper. 2. Materials and Methods The technique employed is the kinetic clotting test, as described by Nosé et al. [11]. Basically, the test consists of the simultaneous filling with donor blood of eight cells containing the candidate material and eight cells containing the control material (Silastic 372).2,3 The cells are sequentially opened at predetermined intervals to follow the kinetics of clot formation. The test cells consist basically of two sheets of the material under study, enclosed within an acrylic chamber (fig. 1). Blood is drawn in between the material sheets by aspirating saline from the acrylic chamber as shown in figure 2. The blood comes directly from the donor dog, contacting only the venous puncture needle and a short segment of 2 Dow Chemical Co., Midland, Mich. 3 Certain commercial materials and instruments may be identified in this publication in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the equipment or instruments identified are necessarily the best available for the purpose. (a) 11/2 CELL FILLING TECHNIQUE TWO ML OF SALINE IS WITHDRAWN FROM AROUND TEST SAMPLES DRAWING BLOOD BETWEEN THE TWO SAMPLE SHEETS FIGURE 2. The cell filling technique. Two ml of saline are withdrawn from around the test sample, resulting in blood being drawn between the test sample sheets. Afterward, they are placed in a slowly rotating device to prevent sedimentation. Two cells, one with Silastic and one with the test material, are opened every twenty minutes over a two-hour period. The clot formation is ascertained using two techniques. The first is the direct measurement of the amount of clot formed. This is done by determining the dry weight of the clot, normalized by the amount of blood in the cell. The actual values of normalized clot weight are plotted versus time. The difference between the clot weights obtained with the test material and the Silastic control is also plotted as a function of time; the area under the difference curve provides a quantitative direct index for comparing different materials. The more negative the number obtained with one particular material, the better its blood compatibility. Second, the amount of unclotted blood which remains within the test cell is measured. The content of the cell is immersed in 100 ml of distilled water, where the unclotted blood hemolyzes. The resulting hemoglobin concentration is measured spectrophotometrically at 5400 A. The hemoglobin readings are normalized by the initial amount of blood, and plotted as functions of time. The differences between the amounts of unclotted hemoglobin obtained with the test material and Silastic control are also plotted, and the area under the curve provides an indirect index where positive numbers predict better blood compatibility than Silastic. The test materials and Silastic control sheets were cut 22-in in diameter and soaked in saline overnight prior to use. All test materials were prepared at one time and stored in covered glass jars. The materials used were: 1. HydronR (2-type E hydrophilic polymer, polyhydroxyethyl-methacrylate). 4 2. Purified Natural Rubber-double centrifuged, prevulcanized latex; 5 50 percent solids exposed to a 4 percent formaldehyde solution after curing and then saline rinsed. 3. Segmented Polyether-polyurethane Ethicon 6prepared according to the method of Boretos [1]. 4. Silastic 372, MDX4-4156-vulcanized at 150 °C, coated with NaHCO3, and washed in saline several days before use (control material). 5. Bovine Pericardium-exposed to 4 percent glutaraldehyde solution (24 h) and saline rinsed. Healthy 20-25 kg dogs were fasted overnight and anesthetized intravenously with 300 mg of sodium thiamylal. Six donor dogs were used in total; one dog was used for each test. 3. Results Figure 3a shows the weight of clot formed by Hydron and that formed by Silastic. The difference in the clot weights is shown in figure 3b. The direct index was -3.02 min. The comparison of unclotted hemoglobin is shown in figure 3c. The difference in the amounts of unclotted hemoglobin between Hydron and Silastic (fig. 3d) yields an indirect index of 16.14 min, figures 4, 5, and 6a, b, c, and d show the results for segmented polyurethane, glutaraldehyde treated pericardium and formaldehyde treated natural rubber, respectively. All results are presented as mean values; the bars represent standard errors of the means. The clotting indices for all materials tested are outlined in table 1. |