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Biomedical subjects

W B Bald

Publications and source records attributed to W B Bald.

7 recordsLinked to original sources
PubMed1990

Cooling rate and ice-crystal measurement in biological specimens plunged into liquid ethane, propane, and Freon 22.

Specimens sandwiched between copper planchettes were plunged up to a depth of 430 mm into coolants used for cryofixation. Hydrated gelatin containing a miniature thermocouple was used to mimic the behaviour of tissue during freezing. Gelatin and red blood cells were used for ice-crystal analysis. Ethane produced the fastest cooling rates and the smallest ice-crystal profiles, and Freon 22 produced the slowest cooling rates and the largest crystal profiles. Smaller crystal profiles were often seen in the centre of the specimens than in subsurface zones. The results show that ethane, rather than propane, should be used for freezing metal-sandwiched freeze-fracture specimens by the plunging method, and probably also in the jet-cooling method. They further suggest that good cryofixation could occur at the centre of thin specimens rather than only at their surfaces. Comparison between theoretical and experimental ice-crystal sizes was satisfactory, indicating that where the experimental parameters can be defined then realistic predictions can be made regarding cryofixation results.

Chlorofluorocarbons, Methane↗
PubMed1986

On crystal size and cooling rate.

A theoretical model is proposed which is used to derive a quantitative relationship between the critical cooling rate and average crystal size at any location within a biological specimen of given shape subject to rapid freezing. The model is applicable to the slamming, plunging or spraying methods of cryofixation provided the ice crystal size is at least 5 times greater than the size of the critical nucleus. Complete vitrification of pure water or aqueous solutions is shown to take place at cooling rates in excess of about 3 X 10(6) K/s.

Crystallization↗
PubMed1984

A helium gas probe for use in cryosurgery.

The design and testing of a prototype cryosurgical probe utilizing helium gas precooled with liquid nitrogen are described. An 8-mm-diameter probe produced an ice ball with a diameter of 28 mm after 10 min freezing using a helium gas flow rate of 42 liter/min. This indicated a surface heat transfer coefficient of 0.34 W/cm2 degrees K and temperature of -138 degrees C at the probe tip. Improved performance figures can be achieved using higher gas pressures and flow rates. A helium gas flow system schematic for use with this new type of cryoprobe is also presented. It is claimed that this system will overcome the problems of developing both multiple-tipped probes and small-diameter needle probes for use in cryoanalgesia.

Cryosurgery↗
PubMed1983

Optimizing the cooling block for the quick freeze method.

It is important for future ultrastructural preservation studies to freeze biological specimens as rapidly as possible. Finite element numerical techniques have been used to compare different cooling block materials used in the quick freeze 'slamming' method. It is concluded that a pure silver block at an optimum initial temperature of about 15.6 K will produce the quickest cooling and will give rates approximately 50% higher than if the block were initially cooled to 4.2 K. A copper block will produce the fastest cooling rates when the block is initially at liquid nitrogen temperature of 77.3 K provided the section being studied is taken no more than about 30 microns from the specimen-block interface. Composite metal blocks can further reduce the total specimen cooling time compared to a mono-block at the same initial temperature but the cooling rate near the interface will be slower. Future methods for specifying cooling rates within thin biological samples should be standardized and must be based on a combination of finite element numerical analysis and the measured temperature at the rear surface of the sample. Thermal contact resistance between cooling block and specimen and between specimen and the rear face temperature sensor must be minimal.

Animals↗
PubMed1979

On defining the thermal history of cells during the freezing of biological materials.

An explicit numerical finite difference scheme for defining the thermal history of a cell or group of cells contained within a biological sample of prescribed shape is described. Solutions are presented for three different cases of freezing of biological materials prior to microscopic examination. It is concluded that a surface boundary condition must be specified in any experiment before cell ultrastructure or viability can be related to thermal history or cooling rate.

Animals↗
PubMed1978

A device for the rapid freezing of biological specimens under precisely controlled and reproducible conditions.

The construction and preliminary testing of a device is described which can be used to freeze biological specimens in any cryogenic liquid at temperatures down to the nitrogen freezing point (63 K) and which can operate in the pressure range 1.3 kNm-2 to 1 MNm-2. Ultra-rapid freezing can be carried out in a subcooled cryogenic liquid either hyperbarically or at atmospheric pressure. Slow freezing rates can be achieved by cooling the specimens in a controlled manner in the vapour phase above the liquid bath.

Atmospheric Pressure↗