Development And Preliminary Application Of A New Cryogenic Gripper Of Biological Soft Tissues

Background:Biomechanics is one of the most important approaches to Medical Science Research. During the course of researches and analyses to the mechanical behaviors of biomaterials, the difficulty of stress concentration and specimen damage were happened very common in ordinary fixation of soft tissues. For overcoming this challenge, the cryogenic techniques was first applied into biomechanical field in 1980s, its special way of fixture extremely decreased the possibilities of the concurrent of stress concentration and specimen damage. However, the cryogenic gripping technique was only utilized into high research fields such as Aerospace, Precision Instruments and National Defense because of its higher contents of technology. Not only the technology was uneasy to extend but also its details were seldom opened to the publics. Nowadays, a few developed counties produce cryogenic grippers used into biomechanics, the functions of these products are relatively limited and with high prices. In our research of biomechanical behavior of human knee crucial ligament fascicles, the fixation of free ends of ligaments has been the bottleneck which blocked further development of the research. So the follow objectives were presented.Objectives:For overcome the difficulty of human cruciate ligaments (HCL) free ends'fixation in tesile test and the defect of current cryogenic gripper's function in bath, to develop a new inexpensive soft tissues cryogenic gripper, this can firmly hold tendons and ligaments fascicles either in air or in bath.Methods:1. Design and manufacture of the cryogenic gripper: Reconstructing copper pipes to be a coolant evaporator with use of grip through a novel architecture; Making the gripper working in saline bath by a waterproof and adiathermance cover; Connecting the evaporator to compressor through coolant input and output ducts, establish the coolant circulation keeping a continuous refrigerant function; Using transparent plastic tub, heat pipe and auto-temperature controller; Designing and making the fitments to connect the gripper and the current material test machine; Assemblage of cryogenic gripper, consist homoeothermic bath tub and material test machine.2. Evaluating the function of this cryogenic gripper through these technical characteristics such as cooling speed, cooling time, clamping stability and holding force.3. Performing uniaxial tensile test of fowl feet tendons and porcine knee ligaments fascicles at 37℃in saline bath to further evaluate the gripper's function. According to experimental data export stress-strain equation of the specimens and check the effectiveness of the equation.Results:1. Refrigerated effectiveness evaluation of the cryogenic gripper: the internal temperature of this gripper come below -20℃from common temperature in 5~10 min and still keeping -20℃+1℃during the entire loading course of 120 min.2. Gripping stability evaluation of the gripper: In 20 times repeated loading and unloading, this gripper can firmly grip wet nylon silks (diam. 3mm) specimen with no slippage by 19 times and only once micro-slippage took place at the first loading.3. Holding force evaluation of the gripper: This cryogenic gripper can firmly grip wet nylon silks specimen (diam. 3mm) and its'each time maximum load value was near or above the 50 kg limit of the transducer, maximum holding force is 499.14 N(+1.05 N).4. The gripper was used to hold fowl feet tendons and porcine knee ligaments fascicles, no slippage and rupture took place during 10 repeated load and unload respectively under 60N and 100N.Conclusions:This novel gripper which developed by coolant cryogenic technology and adiathermance waterproof design, its function is effective and reliable. It can firmly grip soft tissues specimen (diam. 0.5mm～3.0mm), its maximal holding force is above 500 N and can work both in air and salt saline bath. The gripper recruited the defect of current soft tissue cryogenic gripper's inability to work in bath and satisfied the demand of tensile test of tendon and ligament fascicles in modeling physiological environment.