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TECHNICAL PAPERS: Cell

# Forced and Natural Convective Drying of Trehalose/Water Thin Films: Implication in the Desiccation Preservation of Mammalian Cells

[+] Author and Article Information
Bingyan Chen, Alex Fowler

Department of Mechanical Engineering, University of Massachusetts Dartmouth, North Dartmouth, MA 02747

Sankha Bhowmick1

Department of Mechanical Engineering, University of Massachusetts Dartmouth, North Dartmouth, MA 02747sbhowmick@umassd.edu

1

Corresponding author.

J Biomech Eng 128(3), 335-346 (Feb 06, 2006) (12 pages) doi:10.1115/1.2187051 History: Received December 09, 2004; Revised February 06, 2006

## Abstract

Trehalose is believed to offer desiccation protection to mammalian cells by forming stable glassy matrices. The goal of the current study was to explore the desiccation kinetics of thin films of trehalose-water solution under forced and natural convective conditions and to investigate the thermophysical state of mammalian cells at the bottom of the thin film. We developed a finite difference model based on the mass and energy conservation equations coupled to the water transport model from the cells. The boundary conditions were obtained from correlations or experimental measurements and the Gordon-Taylor equation was used to predict the glass transition temperature at every location. Results indicated that there are three distinct regimes for drying for both forced and natural convection, characterized by the slope of the moisture content plot as a function of time. Our results also indicate that the surface of the solution reached the glassy state in less than $10min$ for the Reynolds (forced) numbers explored and $∼30min$ for some Rayleigh (natural convective) numbers; however, significant water was trapped at this instant. Larger drying force hastened quicker glass formation but trapped more water. The numerical model was capable of predicting the drying kinetics for the dilute region accurately, but deviated while predicting the other regimes. Based on these experimental validations of the model, the osmotic response of different cells located at the bottom of the solution with orders of magnitude difference in their membrane permeability $(Lp)$ was predicted. The results suggested that extracellular glass formed around cells at the bottom of a trehalose-water solution by the propagation of glass into the solution; however it takes more than an order of magnitude time ($∼7minto>100min$ for forced convective drying) to remove sufficient water to form glass around cells from the time when the first surface glass is formed. This is attributed to low diffusivity of water through the glass. In addition, the water transport from the glassy matrix could be either diffusion or $Lp$ limited. For diffusion-limited transport, lowering the film thickness at the beginning of drying by half almost lowers the drying time by an order of magnitude. In summary, the optimal design of convective desiccation protocols requires accounting for the size of the cell, their membrane permeability $(Lp)$ and the starting thickness of the solution.

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## Figures

Figure 1

Schematic diagram of the cross section of a small droplet of trehalose solution undergoing air-drying. The volume of the droplet is 30μl. The diameter of the droplet, L, is 10mm. The height of the droplet, s(t), decreases over time as water is evaporating. Water flux from the droplet to the air, jw, can be calculated from the water vapor concentration in air at the liquid-air interface, Cv,s, the water vapor concentration in the surrounding atmosphere, Cv,∞, and mass transfer coefficient, hm. Thin film simulation was performed based on dimensional analysis as shown in text.

Figure 2

Schematic diagram of the experimental setup for force convective drying. 1: gas tank; 2: regulator; 3: flow rate meter; 4: channel; 5: test sample. The test sample was dried by blowing nitrogen gas through the channel at different flow rates. The entire flow path was tightly sealed to prevent leakage. Test samples were taken out of the channel periodically and weighed on a microbalance.

Figure 3

Plot of water activity vs water weight fraction

Figure 5

Plot of experimental results and corresponding numerical simulation of the drying rates of a thin layer of trehalose solution with different initial trehalose concentration in two representative cases. (a) Forced convective drying, Re=450; (b) Natural convective drying, Ram=557. Filled diamond (◆) shows the average value of three individual experiments for 0.1M initial trehalose molar concentration; filled triangle (▴) shows the average value of three individual experiments for 0.2M initial trehalose molar concentration; filled rectangular (∎) shows the average value of three individual experiments for 0.5M initial trehalose molar concentration; error bar represents the standard deviation; solid line (—), dashed line (---), and dotted line (⋯) represent the numerical simulation for 0.1M, 0.2M, and 0.5M initial trehalose molar concentration, respectively.

Figure 6

Plot of water weight fraction vs normalized spacial location in the solution film at various times (in min) in two representative cases. (a) Forced convective drying, Re=450. (b) Natural convective drying, Ram=424. The horizontal axis (z∕s0) represents the spacial location (z) in the film normalized by the initial film thickness (s0). Thickness of the film is decreasing with time.

Figure 7

Plot of predicted volumetric shrinkage response of three types of cells at the bottom of trehalose solution film undergoing forced and natural convective drying. Solid line (—) represents the numerical simulation for forced convective drying, Re=450; dotted line (⋯) represents the numerical simulation for natural convective drying, Ram=557.

Figure 4

Plot of experimental results and corresponding numerical simulation of the drying rates of a thin layer of trehalose solution under forced convection and natural convection. (a) Forced convective drying. Filled diamond (◆) shows the average value of three individual experiments in the case of Re=225; filled rectangular (∎) shows the average value of three individual experiments in the case of Re=338; filled triangle (▴) shows the average value of three individual experiments in the case of Re=450; error bar represents the standard deviation; solid line (—), dashed line (---), and dashed-dotted line (-∙-) represent the numerical simulation for Re=225, Re=338, and Re=450, respectively. (b) Natural convective drying. Filled rectangular (∎) shows the average value of three individual experiments in the case of Ram=424; filled diamond (◆) shows the average value of three individual experiments in the case of Ram=492; filled triangle (▴) shows the average value of three individual experiments in the case of Ram=557; filled circle (●) shows the average value of three individual experiments in the case of Ram=663; error bar represents the standard deviation; solid line (—), dotted line (⋯), dashed-dotted line (-∙-), and dashed line (---) represent the numerical simulation for Ram=424, Ram=492, Ram=557, and Ram=663, respectively.

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