|Fig. 11: First Commercial Ultramicrotome Porter-Blum MT1, 1953 (Sorvall Inc., Norwalk, Conn., U.S.A.)|
The hand-operated Porter-Blum microtome was based on the principle of mechanical advance of the specimen to the knife. Each full turn of the operating handle moved the cantilever arm through one complete cutting cycle, and the only control of the cutting speed was the rate at which the handle was turned.
A specimen block was fixed to the front of a cantilevered arm, the other end of which was mounted on off-set pivots so that the arm was free to move both horizontally and vertically. The end of the arm carrying the specimen block followed a guided path and was displaced sideways during the return stroke thus eliminating possible damage to the tissue block. In the upward stroke, the arm was deflected to one side to avoid contact between the specimen and the knife.
The specimen arm was moved towards the knife by means of a vertical pivot arm whose lower end rested on a micrometer screw and was advanced automatically in regular increments from 25 nm to 500 nm between each cutting stroke.
The microtome was originally designed for the use of glass knives. The suspension of all mechanical components was self-compensating so that static friction had been reduced.
|Fig. 12: Ultramicrotome according to F. J. Sjöstrand, 1953 (LKB, Stockholm)|
In contrast to the Porter-Blum microtome the Sjöstrand ultramicrotome likewise completed in 1953 did not feature a mechanical precision advance.
A bearing housing was mounted on a solid stand in which a high-precision rotor revolved. At its rear side two solid steel shafts were fixed whose ends were connected with a third shaft via a crosspiece. This was passed eccentrically through a bore at the rotor and carried the specimen at the free protracting front. Near the crosspiece the shaft was enveloped by a heating coil of low power. When current was supplied, the rod was heated slowly, it expanded and moved the specimen in the direction of the knife.
An electric motor revolved the rotor once every second whereupon the specimen touched the knife and returned it to the starting point.
A housing protected the steel shafts from air circulation. The electric switching elements were installed at the front side of the housing.
Specially sharpened razor blades were mainly used as knives; these were clamped in a universally adjustable support.
|Fig. 13: Ultratome 1, 1960 LKB|
This further advanced ultramicrotome was characterized by a simple design of the specimen arm that allowed for a sturdy design insensitivity to damage by rough handling.
The specimen arm consisted of a long hollow steel rod connected at one end to the foundation block by two flat phosphor bronze leaf springs in such a way that it was free to move in a vertical plane but not horizontally. A moving coil motor connected to the specimen arm lifted the arm to the top of its stroke. Cutting rates variable between 1 mm to 20 mm per second could be adjusted.
To achieve the desired clearance between the specimen block and the knife on the return stroke, the knife was retracted by applying an electromagnetic force to the platform carrying the knife stage. When the specimen arm had cleared the knife edge on its return stroke, the magnetic action was released and the platform returned to its original position.
The ultra-fine advance was achieved by the fine control of a heating coil which caused the specimen arm to expand towards the knife in regulated amounts from 5 nm to 100 nm between each stroke.
Two independent mechanical controls regulated the coarse and fine advances of the knife. Adjustments for rotating, tilting and lateral movement of knife and specimen were made. The specimen arm could be operated automatically, in single strokes or by a remote mechanical control.
A binocular magnifier could be pivoted about the cutting point.
|Fig. 14: Reichert Ultratome Om U1, 1954 (according to H. Sitte, 1953)|
This ultramicrotome operates according to entirely different principles. Expensive and sensitive precision elements were deliberately replaced by simpler parts.
A brass rod carrying the specimen at its front end was soldered horizontally on a massive copper block which was rigidly fastened to the base. One end of the specimen arm was connected to the feed system and mounted on bearings to allow movement in all directions. The arm movements were controlled by a motor remote from the instrument to avoid the risk of vibrations. Porter's high precision cardan joint was replaced by a simple flexible component. There was now a continuous mechanical connection between the specimen and the stand.
The thermal advance was effected by a 25 W lamp heating a small block of metal which moved the specimen arm towards the knife at regular increments between 0 and 100 nm per stroke as it expanded.
Due to the high temperature capacity the advance remained linear within a large range and was independent of thermal vibrations.
The knife stage featured adjustments similar to those of other microtomes, enabling the knife, as the specimen holder, to be moved in several directions.
|Fig. 15: Philips-Ultramicrotome|
Commercial production 1956-1960
This ultramicrotome differed from classical ultramicrotomes by the way the specimen was moved. By magnetostriction of the nickel rod which carried the specimen it was ensured in a remarkably simple way that the knife did not scrape the specimen surface on the return stroke. Haanstra was the first to describe this principle. The magnetising current switched on periodically produced at the same time the heat necessary for the thermal expansion. The rod on which the specimen was mounted was magnetised after each cutting stroke before it moved upwards again.
The up- and down movement was effected by mounting the specimen arm on a spring connected to the nickel core.
This ultramicrotome had no bearings or lubricated surfaces.
|Fig. 16: Ultramicrotome according to B. von Borries (state of development spring 1954)|
This ultramicrotome consisted of a solid U-shaped cast iron body onto the front of which the guidance for a stable knife-support was attached by casting. The specimen was fixed on one end of a rigid thermally expandable rod which at the other end was attached to the housing by means of a spring joint. The specimen carrier rod penetrated through the eccentric bore of a bearing in the centre of which a cord pulley acted.
Polished razor blades or glass edges which could be adjusted in four independent directions were used for cutting. In the case of mechanical advance the user had the free choice of selecting the speed between knife and specimen.
|Fig. 17: Sartorius Ultramicrotome according to B. von Borries 1956|
In the course of the further development the lower spring joint was replaced by a non-positive ball- and socket joint. A bore was drilled into the leading bolt and a sliding ruler was screwed on. A coarse drive enabled the specimen to be quickly advanced near the knife edge.
The specimen carrier rod could be moved by a forked lifting arm connected with the specimen rod by two cable lines (motor microtome). The movement of the specimen was actuated by a curved disc designed in such a way that it stopped in front of the razor edge and was advanced while cutting at a speed of only 1 mm per second. This speed was found useful during practical work.
|Fig. 18: Trüb-Täuber Ultramicrotome according to D. Danon and E. Kellenberger, 1954|
Like in the Porter-Blum microtome the advance was mechanical but different to this the specimen block was rotated in a plane perpendicular to the knife. The geared-down lever was actuated from this plane.
The advance was effected by the movement of a micrometer screw which pushed an inclined plane forward; the angle of inclination being adjustable.
The advance lever rotated about an axis mounted thereon. In this way the axis could receive the value adjusted for the advancement. The advance could be controlled at uniform steps of 6 to 50 nm, and easily changed to multiples of this value during operation. The same handwheel operated the movement of the specimen lever and the advancement.
The knife holder was fixed on an adjustable slide.
The microtome was insensitive to mechanical and thermal effects.
|Fig. 19: Leitz Ultramicrotome according to Fernández-Morán, 1953|
A massive rotor located in a U-support guided the specimen free of friction and insensitive to vibrations. Due to the high stability it was possible to drive the rotor by a motor. The short stable steel rod for the advance enabled the specimen to be fixed under thermal insulation. It was possible to control the cutting speed continuously.
The heating rod for thermal expansion was diametrically put into the steel rotor. Turning the rotor caused the specimen to describe a circular path. The knife was advanced to the path by means of the support. During each turn thermal expansion lifted the specimen by a small amount beyond the initial circular path and a section was cut.
Besides the common glass knives, diamond knives of superior quality were used. Diamond knives for ultramicrotomy were introduced by Fernández-Morán in 1953.
|Fig. 20: Ultramikrotome Mark 2, since 1970 (KENT CAMBRIDGE MEDICAS - LKB)|
The ultramicrotome followed Mark 1 built by Huxley in 1957.
Operation and advance of Mark 1 were mechanical and its performance was achieved partly through close attention to details of design and through the incorporation of several special features.
The specimen arm was connected by an adjustable wire suspension to the operating lever and was hinged by flexible steel strips or wire connection to the advance mechanism. A micrometer screw advanced the specimen automatically in steps from 0 to 150 nm between each stroke. The arm was raised to the top of the cutting stroke by lifting the operating lever and was then allowed to drop under the action of gravity. An oil-filled dash-pot controlled the rate of fall past the knife edge. The specimen arm was displaced sideways during its return movement.
Mark 2 operates according to the same principle, however a motor has been installed. The motor lifted the specimen arm to the top of its stroke and was then turned off during the downward cutting stroke to avoid vibration. The ultrafine advance could be disconnected, and manual cuts of 0.5, 1 or 2 Ám could then be made.
Shifting and moving mechanisms were completely eliminated at the specimen arm during cutting in order to avoid vibrations.