A Note on "Homogenizers" for Tissue Brei Preparation

Dounce el al. (1) have described a new ball type "homogenizer" that is considered to be more gentle and effective than the device described by Potter and Elvehjem (2). The purpose of this note is to show that the shearing fields actually obtained with homogenizers operated in an up-and-down manner (3, 4) are much greater than those obtained when the pestle is rotated at the speeds generally prescribed (ca. in which G is the rate of shear in see-l, Rj and R2 are the radii of the inside of the tube and of the pestle respectively in centimeters, and n is the pestle speed in revolutions per second. In the instrument described (2), R~ = 0.7 era., R1-R2 ffi 0.0115 era., and the speed was 1200 R.p.~r. The rate of shear is found by Equation (a) to be 7600 sec.-L The percentage of the grinding time, per cent T, spent by any one particle in the shearing space is given by the ratio of the volume of fluid in the shearing space to the total volume: Per cent T ~r(Rl + R,)(RI-R,)L X 100 (b) V * Work performed under USAEC Contract No. W-7405-eng-26. in which L is the length of the pestle parallel to the tube wall, and V is the total brei volume. When V-15 ml., and L-0.65 em., the values for the instances given are 0.22 per cent of the time, or 0.53 second during a 4 minute run. The instrument of the type described by Dounce et al. (1) gives a much higher rate of shear even when the pestle is moved slowly. The rate of shear is di~-cult to evaluate, however, owing to a continuous rate variation Caused by the fluid flowing between a cylinder and a sphere. Furthermore, the annular width is so small that the diameter of the suspended particles is an appreciable fraction of it, and the fluid is moving at such a rapid rate that turbulent flow occurs. It should be emphasized that the values for the shearing fields derived here are only approximations since (a) the rate of shear does not vary linearly from the midannular point to the wall in the vertically moving pestle device, (b) the brei may contain irregularly shaped particles of many different sizes and hence may not behave as a perfect Newt0nian fluid, (c) the suspended particles may create microturbu-lences which are difficult to treat, …