US3447594A - Temperature control for an ultra microtome - Google Patents

Description

June 3, 1969 E. H. ANDREWS 3,447,594

TEMPERATURE CONTROL FOR AN ULTRA-MICROTOME Filed April 19, 1967 Sheet of a June 3, 1969 E. H. ANDREWS 3,447,594

TEMPERATURE CONTROL FOR AN ULTRA-MICROTOME Filed April 19, 1967 Sheet 2 of a 5 I4 8 4 9 ii, I? f3? ii-ii2 133;?? 3 3-5 IO 2 A. v it June 3, 1969 E. H. ANDREWS 3,447,594

TEMPERATURE CONTROL FOR AN ULTRA-MICROTOME Filed April 19, 1967 Sheet 3 of s F|G.5. loo

-25 5O --75 -IOO -|25 -l5O United States Patent 3,447,594 TEMPERATURE CONTROL FOR AN ULTRA- MICROTOME Edgar Harold Andrews, 14 Glebe Road, Welwyn, Hertfordshire, England Filed Apr. 19, 1967, Ser. No. 632,024 Int. Cl. F25b 29/00, 13/00 US. Cl. 165--2 6 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a cooling apparatus for scientific instruments and in particular, but not exclusively, to a specimen freezing device for an ultarmicrotome.

Solid polymers, for example, frequently possess microstructural textures on a scale which can be resolved only in the electron microscope. This is true also of many plastics and rubbers and mixtures as well as semi-crystalline polymers, which constitute one of the largest and most important classes of polymeric solids.

None of the methods so far proposed assist in revealing the true disposition and order of micro-structural entitles in the bulk of such materials in the electron microscope, and are therefore of limited usefulness. However, in accordance with the techniques adapted by the cytoligist who studies the internal structures of biological tissue by means of ultra-thin sections of stained and embedded material by transmission electron microscopy, and by the use of apparatus hereinafter described, successful results may be obtained.

The invention consists in a method of preparing ultrathin sections of solid polymers, filled plastics and mixture of plastics, rubbers and the like, comprising attaching a speciment of material to be sectioned to the specimen head of an ultra-microtome and on the one hand conducting heat away from the specimen at a rate which is in excess of that required to maintain the specimen at a desirable pre-determined sub-zero sectioning temperature, and on the other hand applying to the specimen head heat which is controllable as to its rate of flow to the specimen to maintain the specimen at the said desirable pre-determined sectioning temperature.

The invention further consists in cooling apparatus for carrying out the method as set forth in the preceding paragraph comprising a heat sink which is maintained at a temperature below the desirable pre-determined sectioning temperature, heat conducting means between the specimen head and the said sink for conducting heat away from a specimen in the head to the said heat sink, a source of heat which is adjustable as to its temperature, heat conducting means between the specimen head and the said source of heat for conducing heat to a specimen in the head from the said source of heat.

The invention still further consist in apparatus as set forth in the preceding paragraph including heat conducting means for conducting heat away from the sectioning knife of an ultra-microtome to the said heat sink, and a further source of heat which is adjustable as to its temperature and heat conducting means for conducting heat to the cutting knife from the said further source of heat.

The invention still further consists in apparatus as set forth above in which the heat sink comprises a vessel containing a liquid coolant, While the source of heat comprises an electric wire wound heater.

The apparatus as above set forth may be provided with thermocouples associated with the specimen head and the cutting knife, connected to indicating means, and temperature adjusting means.

The accompanying drawings show, by way of example only, one embodiment of the invention in which:

FIGURE 1 is a general perspective View of the cooling device in use with an ultra-microtome,

FIGURE 2 is a plan view, part in section, of the cooling device of FIGURE 1,

FIGURE 3 is a section on the axis of the specimen head of the ultra-microtome and head cooling member of the device, and through the sectioning knife and cooling member of the device,

FIGURE 4 is a cross section through the head cooling member of the device, while FIGURE 5 is a graph showing the relationship between the voltage applied to the electric heater and the temperature of the specimen head.

The normal specimen chuck of the ultra-microtome is replaced by a specimen head comprising a short cylindrical member 1 of thermally insulating material such as Tufnol or PTFE, and to the end of this a conducting bar 2 and a specimen nose-piece 3 are attached. The conducting bar may be conveniently made of copper rod of diameter, and have attached thereto or formed integral therewith a short specimen nose-piece 3.

The conducting bar 2 is fastened to the cylindrical member 1 by the screws 4, and is thus thermally insulated from the attachment means 5 for the chuck of the ultra-microtome. The rod extends horizontally to one side of the ultra-microtome where it turns downwards into a vessel 6 which contains liquid nitrogen, or other suitable coolant, and acts as a heat sink. The immersed end of the conduction bar 2 carries a number of copper vanes 7 to facilitate heat transfer between the bar and the coolant.

The copper bar 2 is extended a short distance on the opposite side of the specimen nose-piece 3 to the vessel 6, and is counter-weighted to avoid torsional stresse in the specimen bar of the ultra-microtome 5.

The whole of the exposed parts of the conduction bar 2 are thermally insulated by a cladding 8 of expanded polystyrene. The only parts left exposed are the end immersed in the coolant, and the nose-piece, which protrudes through the insulation and carries the specimen, the nose-piece being kept as small as possible to reduce formation of ice due to condensation around the specimen.

The vessel 6 containing the coolant, is also clad all round in expanded polystyrene to prevent as far as possible entry of heat from the atmosphere.

Cooling of the knife 9 is also important, to avoid undue warming of the specimen, as each section is cut and to prevent the tendency of sections to concertina at the knife edge. There is provided a copper block 10 which rests on the inclined upper surface of the knife 9, and has two wings 11, of sheet copper, which cover the sides of the knife 9 right up to the cutting edge, and also form the sides of a reservoir 12 for the knife fluid.

Heat is conducted from the knife cooler to the coolant vessel 6 by a conduction bar 13, formed by a shank of twisted copper wires. The copper block 10 and the bar 13 are clad in expanded polystyrene 8 in the same manner as for the specimen cooler, the conduction bar 13 being exposed only where it enters the coolant vessel 6.

In order, in accordance with the invention, that the temperature of the specimen and the knife should be adjustable to a predetermined value, the conduction bar 2 of the specimen head and the conduction bar 2 of the specimen head and the conduction bar 13 of the knife are provided with electric heating coils 14 and 15 respectively. These coils dissipate a maximum of 120 Watts at volts and are controlled manually by means of an autotransformer in the primary circuit of a step-down transformer. The specimen head heater and the knife heater are controlled separately by the two control knobs shown on the control unit 16 in FIGURE 1.

The temperatures of the specimen and knife are measured with copper-constantan thermocouples attached respectively to the nose-piece, close to the specimen, and to one of the knife-cooler wings close to the cutting knife. These thermocouples are connected to the control unit 16 by the wires 17 and 18 respectively, and the voltages and hence the temperatures produced can be read alternatively on the meter 19, by operating the switch 20. Although the temperatures so measured are not actually those of the specimen itself or that of the surface of the glass, they can be used consistently to determine and to repeat the optimum conditions found empirically.

Using liquid nitrogen at l96 C. as coolant the specimen temperatures achieved are given in FIGURE 5 as a function of the voltage supplied to the heating coils. Thermal equilibrium is obtained about 15 minutes after filling the coolant vessel, although slow temperature drifts need to be occasionally corrected by adjustment of the heater inputs. No adjustments, however, are usually necessary during the period occupied in cutting a single ribbon of sections.

Uniform cutting may be obtained with the apparatus working in accordance with invention down to instrument settings of 300 A., providing, of course, that a good knife is used. The knife edge may be lubricated with ethylene glycol or a glycol-water mixture without filling the knife reservoir 12 completely. The specimen surface is kept clear of ice crystals by the cutting action, and the knife edge by the presence of the ethylene glycol. However, the specimen area is best brushed periodically with a camelhair brush.

It may be seen from FIGURE 5 that with a change of heater voltage from zero to full voltage, the temperature can be adjusted to any predetermined sub-zero value to 150 C. with a coolant temperature of 196 C. Specimens of different materials require different sectioning temperatures, for example, high density polyethylene requires a temperature between 70 and 100, low density polyethylene -80 to -120, polypropylene -140, and polychloroprene and natural rubber l or lower. In all these cases the temperature of the knife may be in the region of -35 C.

It may therefore be appreciated from the foregoing description that by the conduction of heat from the specimen at a rate in excess of that required to maintain the desired cutting temperature alone, and by conducting heat to the specimen in a controlled manner, the sectioning temperature may be precisely maintained at any desired value within the range of temperatures provided by the apparatus.

Although the invention has been hereinbefore described particularly in relation to an ultra-microtome, the invention is not restricted to such use, and may be usefully applied to other scientific instruments, where a controllable sub-zero temperature is required to be precisely maintained. Also, it is to be understood that the above description is by way of example only, and that details for carrying the invention into effect may be varied without de parting from the scope of the invention claimed.

What I claim is:

1. Method of preparing ultra-thin sections of solid polymers, filled plastics and mixtures of plastics, rubbers and the like comprising attaching a specimen of the material to be sectioned to the specimen head of an ultramicrotome, and on the one hand conducting heat away from the specimen at a rate which is in excess of that required to maintain the specimen at a desirable pre-determined sub-zero sectioning temperature, and on the other hand applying to the specimen head heat which is controllable as to its rate of flow to the specimen, to maintain the specimen at the said desirable pre-determined sectioning temperature.

2. Cooling apparatus for cooling a specimen of material to be sectioned while upon the specimen head of an ultra-microtome comprising, a heat sink remote from the specimen head and comprising a receptacle containing a liquid coolant which boils at a temperature below that of a desired predetermined sub-zero sectioning temperature, a metallic conduction bar, a specimen nosepiece upon the bar, intermediate its ends, and attachable to the specimen head of an ultra-microtome with one end of the bar immersed in the coolant in the receptacle, for conducting heat from a specimen on the nosepiece to the heat sink, an electric heating coil upon the end of the bar remote from the heat sink, a source of electric current connected to the electric heating coil, and current flow adjusting means for adjusting the value of current passing through the heating coil to adjust the temperature of the 30 specimen upon the nosepiece to said desirable predetermined sectioning temperature.

3. Cooling apparatus as claimed in claim 2 comprising in addition a flexible metallic conduction bar one end of which is immersed in the coolant in the receptacle, and the other end of which is connected to a metal block adapted to rest upon the knife of the ultra-microtome, an electric knife heating coil in thermal contact with the block and connected to the source of electric current, and current flow adjusting means for adjusting the value of current passing through the knife heating coil to adjust the temperature of the knife to a desirable predetermined sub-zero temperature.

4. Cooling apparatus as claimed in claim 3 having thermocouples associated with the specimen head and the sectioning knife, electric current indicating means and electrical connections between the thermocouples and the indicating means.

5. Cooling apparatus as claimed in claim 4 in which the conductor bars and the receptacle are each covered with a heat insulating material.

6. Cooling apparatus as claimed in claim 2 in which the heat conducting means and the vessel are covered with a heat insulating material.

FRED C. MATTERN, 111., Primary Examiner.

M. ANTONAKAS, Assistant Examiner.

US. Cl. X.R.

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