Cryogenic Ultramicrotome Seal

The invention relates to thermally insulating cryogenic ultramicrotome seals.

Cryogenic microtomes and ultramicrotomes generally include an enclosed cryogenic chamber which is cooled by a cryogenic liquid coolant, such as liquid nitrogen, to control the interior of the cryogenic chamber to as low as -196.degree. C. Within the cryogenic chamber are typically located a knife holder and a specimen holder which are moveable relative to one another to produce ultrathin slices of the frozen specimen. Such a known structure can be found in U.S. Pat. No. 3,828,571, the disclosure of which is expressly incorporated herein by reference. These ultrathin samples are necessary for examination of the specimen with, for example, an electron microscope.

Relative movement between the knife and the frozen specimen within the microtome cryogenic chamber can be accomplished by mounting the knife or specimen or both on microtome arms which extend through holes in the cryogenic chamber to the exterior of the chamber where the arms can be manipulated at room temperature. When such microtome arms pass through apertures in the cryogenic chamber, a thermally insulating seal is desirable in order to minimize evaporation of cryogenic liquid contained within the cryogenic chamber and to minimize thermal gradients within the cryogenic chamber which may deleteriously affect the temperature stability of the frozen specimen or knife, or both. At the same time, such a cryogenic seal must not interfere excessively with the precise movement of the microtome arm.

The present invention is a seal for use in a cryogenic microtome which allows a microtome arm to pass from the interior to the exterior of the cryogenic chamber, thereby providing a thermally insulating seal without excessive interference with precise movement of the microtome arm.

The seal has a labyrinth form and includes a seal container with a plurality of baffle inserts therein, substantially fixed relative to the container. Each of the baffle inserts has an aperture which is substantially aligned with the aperture in the cryogenic chamber and which has a perimeter which is greater than the perimeter of the microtome arm. The baffle inserts are spaced apart to define a plurality of baffle chambers within the seal container. Contained and moveable within each baffle chamber is a baffle membrane through which the microtome arm passes in sliding contact. Fixed to the seal container is a baffle wall plug which has a periphery configured for press fitting within the aperture of the cryogenic chamber, thereby allowing the cryogenic seal surrounding the microtome arm to be attached to the cryogenic chamber. The baffle inserts and baffle membranes together form a thermally insulating labyrinth seal between the interior and exterior of the cryogenic chamber. The movement of the baffle membranes within the baffle chambers formed by the baffle inserts combined with the sliding engagement between the baffle membranes and the microtome arm, allow relatively unimpeded manipulation of the microtome arm from the exterior of the cryogenic chamber.

The baffle inserts, baffle membranes and cryogenic arm are preferably of thermally insulating material, while the seal container is preferably thermally conductive. The baffle membranes can be constructed of, or coated with, anti-static material in order to eliminate the deleterious effects of static electricity within the seal. A low wattage heater can be attached to the exterior of the seal chamber which serves to maintain the interior of the seal in a relatively frost-free condition further promoting ease of manipulation of the microtome arm passing therethrough.

FIG. 1 is an exterior view of the cryogenic microtome seal of the present invention.

FIG. 2 is a view taken from section 2--2 of FIG. 1.

FIG. 3 is a view taken from section 3--3 of FIG. 1.

FIG. 4 is an exploded view of the cryogenic microtome seal of the present invention.

FIG. 5 is a view taken through section 5--5 of FIG. 1.

FIGS. 6A and 6B are views of the seal container used in the present invention.

FIG. 7 is a baffle insert used in the present invention.

FIG. 8 is a baffle membrane used in the present invention.

FIGS. 9A and 9B are views of a baffle wall plug used in the present invention.

Referring to FIGS. 1-3, the cryogenic microtome seal 10 of the present invention is shown in place within aperture 11 of cryogenic chamber 12. In FIG. 1, the low temperature interior of chamber 12 is to the left and the room temperature exterior of chamber 12 is to the right.

Seal 10 is attached to chamber 12 by use of baffle wall plug 13 which is press fitted or otherwise attached within cryogenic chamber aperture 11. The exterior periphery of baffle wall plug 13 is configured to be complementary in shape to the interior periphery of cryogenic chamber aperture 11. Passing through seal 10 from the interior to the exterior of chamber 12 is microtome arm 14 which is preferably constructed of thermally insulating material, for example KEL-F 81 brand plastic material, available from 3M Corporation.

Seal 10 is enclosed within seal container 16 which is preferably constructed from thermally conductive material such as aluminum, copper, or brass. Attached to the exterior of container 16 is low wattage electrical heating element 17 which is connected to a suitable source of electrical power (not shown) by leads 18. Heater 17 serves to maintain the interior of seal 10 in a defrosted condition thereby facilitating the manipulation of microtome arm 14.

Contained within seal container 16 are baffle inserts 19 and baffle membranes 21, the placement of which will be explained in more detail below with respect to FIGS. 4 and 5. Within the interior of cryogenic chamber 12, microtome arm 14 can be used in a manner well known to those skilled in the art to support and manipulate a frozen specimen or a microtome knife.

Referring to FIG. 4, which is an exploded view of the present invention, within seal container 16 are a plurality of baffle inserts 19 interspaced with a plurality of baffle membranes 21. Baffle inserts 19 are configured to be press fitted within the interior of seal container 16 and, when assembled, are substantially fixed relative to seal container 16. Baffle wall plug 13 is affixed, by use of an appropriate adhesive, thermal bonding, or the like, to the left-most baffle 19. Baffle membranes 21 are slidably mounted on microtome arm 14 and are moveable relative to baffle inserts 19 and seal container 16. A section of the interior of cryogenic seal 10 is shown in FIG. 5 which is a view taken through section 5--5 of FIG. 1. Individual baffle inserts 19 are press fitted in spaced apart relationship within seal container 16 to form baffle chambers 22. Baffle inserts 19 are moveable within baffle chambers 22 between baffle inserts 19. Three to five pairs of inserts 19 and membranes 21 have proven acceptable, however any number can be used.

FIGS. 6A and 6B are views of seal container 16. Container 16 is preferably thermally conductive and can be stamped aluminum which is then anodized to prevent surface corrosion. Container 16 has a central aperture 23 which is large enough to accommodate microtome arm 14 (see also FIG. 3) without contact. Seal container 16 is formed with side walls 24. Other suitable materials for container 16 include copper or brass, although other thermally conductive materials are also suitable.

Referring now to FIG. 7, baffle insert 19 is shown. The periphery of baffle insert 19 is configured to fit snugly within the interior of seal container 16 and when assembled, individual baffles 19 are preferably substantially fixed relative to seal container 16. Baffle inserts 19 include an aperture 25 therein through which microtome arm 14 passes without contact when baffle inserts 19 are assembled as shown in FIG. 4. Baffle inserts 19 are preferably thermally insulating and can be made from a closed cell material with a smooth surface such as polystyrene sheet, although other thermally insulating materials are acceptable.

Referring to FIG. 8, the details of baffle membrane 21 are shown. Baffle membrane 21 includes aperture 27 therein which is dimensioned to engage microtome arm 14 (see also FIG. 3) with sliding contact. Aperture 27 is configured to be complementary in shape to the periphery of the cross-section of microtome arm 14. A circular shape is preferred but other geometric shapes are also acceptable. Baffle membrane 21 is constructed of a thermally insulating material and is preferably antistatic. Suitable materials include anti-static treated mylar, mylar coated with anti-static material such as iron oxide, or functionally equivalent structures.

Finally, with reference to FIGS. 9A and 9B, baffle wall plug 13 is shown. Baffle wall plug 13 includes aperture 28 therein through which microtome arm 14 passes without contact when seal 10 is assembled (see also FIG. 2). The outer periphery of baffle wall plug 13 is configured to be press fitted within aperture 11 of cryogenic chamber 12 (see also FIGS. 1 and 2). Baffle wall plug 13 is preferably made from a closed cell plastic foam material such a polystyrene or polyurethane.

While the present invention has been described with reference to a single preferred embodiment, one of ordinary skill in the cryogenic microtome art will understand that modifications, additions or deletions to the invention can be made, without departing from the spirit and scope of the claimed invention.