Viscosity of Cryoprotective Agents Near Glass Transition: A New Device, Technique, and Data on DMSO, DP6, and VS55

Abstract

The low strain-rate viscosity of glass-forming cryoprotective agents (CPAs) in the vicinity of the glass transition is studied experimentally. Data on the mechanical behavior in this regime is necessary to the long-term goal of developing planning tools for cryopreservation via vitrification (vitreous means glassy in Latin); such tools will provide guidelines for reducing thermal stress with its devastating effects. While the flow behavior of some glass-forming CPAs is well documented in the literature for the upper part of the cryogenic temperature range (where the CPA has a comparatively low viscosity), it is the flow behavior near the glass transition temperature (where the CPA behaves as nearly a solid with an extremely high viscosity) which is critical to the analysis of stress that develops in the cryopreserved material. If the elevated viscosity limits the material’s ability to flow—in order to accommodate the thermal strain resulting from large temperature gradients, especially at the high cooling rates necessary to form glass—structural damage may follow. Information on the behavior of the CPA in the lower part of the cryogenic temperature range is largely unavailable. A new measurement device is presented in this study, in which a solid rod is pulled from a long narrow cup containing a CPA, producing an essentially one-dimensional and isothermal field of flow. The viscosity and relaxation time of the CPA is inferred from measurements of the resulting load on the rod when extracted at a constant velocity. The current study reports on experimental data near glass transition of 7.05 M DMSO, a reference CPA solution, and the CPA cocktails VS55 and DP6.

Appendix Uncertainty analysis

Following standard practice [16], the uncertainty in this procedure is estimated as:

$$\delta F\left( {x_1 ,x_2 , \ldots .,x_i } \right) = \sqrt {\sum\limits_i {\left( {\frac{{\delta F}}{{\delta x_i }}\delta x_i } \right)^2 } } $$

(A1)

where x _i are the independent variables. In the current study, the independent sources of uncertainty are the observed steady-state load, P _ss , the outer radius of the stainless steel rod and inner radius of the brass sample cup, R ₁ and R ₂, respectively, the length of the stainless steel rod submerged in CPA, L, and the velocity that the rod is extracted, v _o, which is translated to a strain rate.

Uncertainty in load cell measurement is caused by nonlinearity (±0.05% of full scale), hysteresis (±0.05% of full scale), non-repeatability (±0.05% of full scale), and temperature shift (±0.0014%/°C of actual load). Uncertainty in radii measurement is estimated as 0.01 mm. Uncertainty in L originates from the uncertainty in the CPA volume injected into the CPA chamber; an uncertainty of 0.29 mm is estimated when using a 1 mL syringe. Another source of uncertainty in L is the gradual extraction of the upper rod from the CPA, which may be as much as 2.5 mm over the duration of the experiment.

Uncertainty in temperature measurements is introduced by A/D conversion (22 bits at 0.333 Hz) in the data acquisition module, cold-junction compensation, and the quality of the thermocouple material. The combined effect of these uncertainties is estimated as ±0.8°C.

Uncertainty was calculated for each experiment based on equation (A.1) and the above data. Uncertainty in viscosity calculations based on experimental data ranged from 2.9 and 8.4% in 7.05 M DMSO experiments, between 2.3 and 10.8% in VS55 experiments, and between 3.6 and 11.1% in DP6 experiments.