Med. & Biol. Eng. & Comput., 1978, 16, 226-228

Technical note Apparatus for uniaxial testing of soft human tissues in vitro Keywords--Biological tissue. Mechanical properties 1 Introduction IN investigations of the mechanical properties of soft biological tissues, the complex nature of the response and the need to maintain a viable test specimen require facilities not available in commercial devices. Considerable development of apparatus designed primarily for investigating the contractile properties of h u m a n skeletal muscle and the tensile properties of soft h u m a n tissue has taken place over the past two years. This technical note lists the specifications of the apparatus and outlines its main features. For a detailed description and discussion of its development and use, the reader is referred to STEVENS (1976a). A manual describing the mechanical construction and electronic circuits in detail is available (STEvENs, 1976b) and may be of value for those intending to construct similar apparatus. Examples of typical results obtained from skeletal muscle biopsies and cervical tissue strips (STevENs, 1976c) are shown in Figs. 1 and 2. The linear motor servo is used to produce the rapid length change in Fig. 1 and the rotary motor servo with force feedback is used to produce the slow ramp force input in Fig. 2.

3 Main features The complete apparatus as assembled for hospital or laboratory use is shown in Fig. 3 and 4, Fig. 4 summarises the main interconnections. A machine developed for cutting cervical-tissue test specimens is shown in Fig. 5. Recordings were made on a fibre-optic oscilloscope (Medelec FOR-4) or X - Y recorder, depending on application, and an f.m. tape recorder (SE Labs. 8-4) if further analysis was required. The main features are as follows: (i) facilities for maintaining tissue at a preset tem2

Stress (N /mm~ )

1

2 Specifications The specifications for the apparatus (Table 1) were based on the experience gained from the use of an earlier apparatus (FAY et aL, t974). Apart from a poor performance of the rotary motor servo at very low rates of input they have been achieved.

I

I

1.5

2.O Extension

ratio

Fig. 2 Force /length curve for a cetvical tissue specimen to a ramp force input 50kN/m z 200ms

Displacement

input

02mm

Stimulation

frequency

input 35 p p s 25 lap s

Fig. 1 Force response of a skeletal muscle specimen to a simultaneous ramp change in stimulation pulse rate and specimen length Received 12th July 1977

Fig. 3 Complete apparatus

0140-0118/78/0721-0226 $1 950/0 9 IFMBE: 1978

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Medical & Biological Engineering & Computing

March 1978

Ringers

Fig, 4 Schematic of Fig. 3 showing interconnections

Supply

Recording Oscilloscope 0

0

0

0

Rotary motor servo

Timer/

Linear mofor servo

Controller

Stimulator

l J

I

~

,~" U

=J r Transducer

_, I :

il

omplifier5

_ ~

-~

I

i

rl l

I

Table 1. Specifications for apparatus for testing the mechanical properties of soft human tissue Specimen size, mm

length width thickness

Environment

Ringer's solution at 2 litres per hour Temperature 37 ~ C + O. 5 ~ C Perfusion with 95% oxygen, 5% CO2 gas mixture

Force limits, N

0-10

Displacement limits, mm

0-50

inputs

(i) short range, high rate ; _ 1 mm, rise time less than 3 ms (ii) long range, l o w rate; _+ 25 mm, 0 . 0 2 -- 10 mm/s (iii) facility for force imputs, with equivalent specifications

Stiffness of linear motor servo system

greater than 15 N / m m ; i.e. less than 0,7 mm movement for maximum force specification

Stimulation

facility for voltage control of stimulation frequency in the range 1 0 - 1 0 0 pulse/s

Ramp generator

t w o inputs to integrator, time constants 10 m s 1 00 s, linear up to a 1V ramp in 30 min, drift less than 0-00001/s

Timer/controller

master patch panel to patch combinations of force, length or stimulation input w i t h respect to time or outputs from specimen; recycling facility for fatigue tests

Medical & Biological Engineering & Computing

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perature, immersed in a suitable solution and perfused with oxygen (ii) trolley to hold complete system with wiring built in (iii) facilities for stimulation of specimen (multiwire and single-wire electrodes) including control of level, pulse width and voltage control of frequency (iv) interchangeable clamps and force transducers to enable testing of different types of tissue (v) rotary motor servo for applying low rates of input

(ix) parts of the apparatus have found use in the uniaxial testing of soft materials and in the provision of controlled mechanical inputs in, for example, in vivo clinical experiments J. C. STEVENS University and Area Health Authority (Teaching) Department of Medical Physics Weston Park Hospital Sheffield SIO 2S J, England N. B. JONES Graduate Division of Biomedical Engineering University of Sussex Falmer, Brighton, England D. F. FAY Dalhousie Medical School Halifax, Canada

Fig. 5 Tissue cutting machine (vi) linear motor servo for applying high rates of input over a limited range (vii) transducer amplifiers for matching the recorded signal to the recording device (viii) timer/controller for automation of experiments to allow patching of force, length had stimulation inputs, controlled by time or output signals from the specimen

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References FAY, D. F., JONES, N. l . , PORTER, N. H. and WOOD, R. A. (1974) Developments in apparatus for dynamic in vitro testing of human muscle. Med. & Biol. Eng. 12, 647-663. STEVENS, J. C. (1976a) In vitro investigations into the mechanical properties of active and passive soft human tissues. D.Phil. Thesis, University of Sussex, England. STEVENS, J.C. (1976b) Manual for apparatus for testing the mechanical properties of soft human tissues in vitro. Internal publication, Graduate Division of Bioengineering, University of Sussex, England. STEVENS,J. C. (1976c) Uniaxial tensile testing of cervical tissue in vitro in relation to dilatation of the cervix. Symposium Biomed. Eng. in Obst. and Gyn., University of Sussex Jan. 1976.

Medical & Biological Engineering & Computing

March1978

Apparatus for uniaxial testing of soft human tissues in vitro.

Med. & Biol. Eng. & Comput., 1978, 16, 226-228 Technical note Apparatus for uniaxial testing of soft human tissues in vitro Keywords--Biological tiss...
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