Simple computer controlled apparatus for zyxwvutsrqponmlkjihgfedcbaZY in vitro mechanical testing of connective tissues T-A. Sikoryn, R.M. Aspden and D.W.L. Hukins Department of Medical Biophysics, University of Manchester, Stopford Building, Manchester Ml3 9PT, UK Rccciwd August 1987, accepted Dcccmhcr 1987 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF ABSTRACT zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA C’onnectivekwea are oi.tcoelaslicand so a serie.5 of experimenkr, at carefully controlled loading rates, is required to determine lheir mechanical properlies. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA This paper describes a simple, inexpensive apparatus for performing these experiments; il i.s based on a ball .screw driven by a microcomputer conlrolled stepper molar. Keywords: Connective tissues, mechanical testing, computrr control INTRODUCTION There is an extensive and growing literature dealing with the mechanical properties of connective tissues- cartilage, fascia, ligament etc.‘,‘. These tissues are viscoelastic, i.e. their mechanical res P onse depends on the rate at which they are loaded . They exhibit KY;erF;;Er;es as creep, stress relaxation and a variety of experiments is therefore required to define their properties. It is important that apparatus used for measuring the mechanical properties ofconnective tissues in vitro should satisfy two requirements: carefully controlled timing, and versatility. Various ingenious devices have been described for performing specific tests (e.g. references 5-9) but these, by their very nature, are not versatile. The simplest way of satisfying the two requirements simultaneously is by computer control. Unfortunately, most computer controlled mechani- cal testing equipment is expensive, because of the accuracy required for some tests on engineering materials. However, biological variability is respon- sible for most of the spread in results obtained from tissues, so that this very high accuracy in measure- ment is not always essential. The purpose of this communication is to describe how a stepper motor, controlled by a microcomputer, can be used as the basis of an inexpensive and simple device for performing mechanical tests on connective tissues. Specimens are subjected to controlled strain rates, and the resulting forces measured by a load cell. However, stress rates could also be controlled by monitoring load cell readings and using them to modify strain rates during an experiment. Computer control not only allows complete flexibility in experimental design but also enables results to be stored, plotted in various formats, and compared with the results from other experiments. Further- more, simultaneous video recording allows Poisson’s ratio effects and structural changes to be measured, Correspondence and reprint requests to Dr D.W.L. Hukins CC‘, 1988 Ruttrrwurth ((i CO (Publishers) Ltd 0141~5425/88/040357-03 $03.00 processed by computer and, together with load cell readings, stored either on disk or on an annotated video tape”. MECHANICAL ASPECTS A stepper motor which generates a torque, T, when it steps throygh an angle, 8, will produce a translation, X, m a lead screw of pitch, p, capable of generating a force, F. This force will be tensile or compressive, depending on the sense of x. These quantities are related by: e/27c = xip and Fx= zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML TO therefore T = FxjQ = Fpl2~ Required values of F can be estimated from the dimensions of tissues and their published mechanical properties. Hence, a combination of values for T and p can be chosen to achieve these requirements. However, it takes a finite time for the motor to achieve stable operating conditions. It is essential that this time should be small compared with the interval between load cell readings, otherwise the timing of the experiment would be unreliable when the motor started or changed direction. These considerations led to the choice of a Sigma Instruments Model 2 1/2235D200/F037 stepper motor”, capable of generating a torque of 1 Nm within a time of about 1 ms, for speeds of less than 1000 steps per second. To minimize backlash, the motor was connected by a toothed belt to a low cost rolled-thread ball screw of pitch 2.5 mm (manu- facturer’s target accuracy 0.07 %)‘I. Since the mo_tor can step in increments of 1.8”, this combination J. Biomrd. Ena. 1988, Vol. IO,.July 357