K. Chinzei, K. Miller, "Compression of Swine Brain Tissue; Experiment In Vivo," in Proc. International Society of Biomechanics Tokyo Congress 97, 1997, p. 318. Compression of Swine Brain Tissue; Experiment In Vivo Kiyoyuki Chinzei and Karol Miller Biomechanics Division, Mechanical Engineering Laboratory, MITI, Tsukuba, Japan Email: chin@mel.go.jp; kmiller@mel.go.jp Introduction Recent developments in Computer-Integrated and Robot-Aided Surgery [1], as well as advances in virtual reality techniques [2], call for closer examination of the mechanical properties of very soft tissues (such as brain, liver, kidney, etc.). The ultimate goal of our research into the biomechanics of these tissues is development of corresponding, realistic mathematical models. Such models would find applications in surgeon training and operation planing systems, based on the virtual reality techniques, registration [3], and control of surgical robots. The presented contribution, intended as a step towards the said goal, shows the experimental results of in vivo compression of swine brain tissue. Materials and Methods In vivo compression of swine brain tissue was performed in a testing stand, as shown in Fig. 1. The main testing apparatus was a two-linear-degree-of-freedom robot equipped with a sensitive load cell, build in our Laboratory. A head of a six-month-old, anesthetized swine was fixed rigidly in the robot base, Fig. 1. An opening in a skull of about 2.5 x 2.5 cm was prepared at the frontal lobe. The robot's cylindrical (10 mm diameter) end effector was placed so that it touched the exposed brain surface. The measurement began when the robot's end effector started vertical descent. The loading velocity was 1 mm/s. The robot movement was controlled and measured precisely by linear displacement sensors. The loading force was measured by a sensitive strain-gauge load cell. The experiment consisted of two stages: loading and stress relaxation. This paper focuses on the results obtained during the loading phase. For compressive strains larger than 35%, measured in vitro [4], we observed unrecoverable damages in the tissue. Therefore, only one loading cycle was performed. The start of the loading phase was indicated by the first non-zero reading of the load cell. The end of loading phase and simultaneously the start of stress relaxation phase was indicated by the point of equalizing of the reading of the vertical displacement. Results Figure 2 shows loading force versus displacement curve. Surprisingly, even for high compression levels, the force-displacement relation is close to linear, with proportionality constant of 0.099 N/mm. Discussion and Conclusions This study presents the results of the in vivo compression of swine brain tissue. Close to linear force-displacement relation was observed. How to use the in vitro experimental results for defining models of brain mechanical properties remains an open question. Further research is needed to determine brain tissue constitutive models, which would incorporate the influence of the blood and cerebrospinal fluid pressure and flow. Acknowledgments The financial support of Japanese Science and Technology Agency and the New Energy and Industrial Technology Development Organization (NEDO) is gratefully acknowledged. References [1] Taylor, R.H., LavallŽe, S. and Burdea, G.C. (ed.), ÒComputer-Integrated SurgeryÓ, MIT Press, Cambridge, 1995. [2] Burdea, G., ÒForce and Touch Feedback for Virtual Reality,Ó Wiley, New York, 1996. [3] LavallŽe, S., ÒRegistration for Computer Integrated Surgery: Methodology, State of the ArtÓ, Computer-Integrated Surgery, MIT Press, Cambridge MA (1995), pp. 77-97. [4] Chinzei K., Miller K., ÒCompression of Swine Brain Tissue; Experiment In VitroÓ, J Mech Eng Lab, 50-4, 1996, pp. 19-28. Fig. 1. Experimental stand Fig.2. Force - displacement curve obtained in vivo