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2025.06.21 Calcium Diffusion in Vials (Earth Sand)

Purpose

For diffusion modelling of calcium crosslinking in the sand alginate gel, we will measure the difference in weight and calculate absorbed calcium chloride volume over different crosslinking times.

Materials & Methods

Materials

100 mM calcium chloride
3wt% alginate solution
30wt% 425 µm earth sand
9wt% CMC
Spatula
2 x 1 mL syringe
12 x 5 mL glass vials
red food colouring
disposable transfer pipette Methods

Make 10 mL of 30wt% earth sand, 9wt% CMC and 3wt% alginate gel by following procedure listed in Bioink Composition Testing
Load 1 mL of the earth sand gel into a 1 mL syringe and extrude into 6 vials
1. ensure there are no bubbles, tap the vial down after extruding to ensure the top surface is flat
Label each vial with 2, 5, 10, 15, 20, and 60 minutes respectively
Repeat steps 1-3 but load pure 3wt% alginate
In 15 mL of 100 mM calcium chloride solution, add ~2 drops of red food colouring and mix
Weigh each vial and record the initial mass
Pipette 1 mL of the red 100 mM calcium chloride solution into each vial
Start a timer for crosslinking based on the labelled time
Remove the calcium chloride solution using a disposable transfer pipette after 2, 5, 10, 15, 20, and 60 minutes of crosslinking (based on the vial)
1. ensure not to disturb the gel
Weigh the vial again and record the final mass
Calculate the volume of calcium chloride absorbed for each time point by using the below formula and a calcium chloride density of 1.007 g/mL
Graph crosslinking time vs. volume of calcium chloride absorbed $\text{Volume}=\frac{\text{Final weight - Initial weight}}{p_{CaCl_2}}$

Results

Figure 1. 60mins, 20mins, 10mins, 5mins, 2mins (left to right) crosslinking with 30wt% gel. Can see slight pink at the surface going from right to left (with increasing crosslinking time). 15mins is missing since it was taken out.

Figure 2. 60mins, 20mins, 10mins, 5mins, 2mins (left to right) crosslinking with alginate gel. Can see increasing pink / solid gels going from right to left (with increasing crosslinking time). 15mins is missing since I took it out already.

Table 1. Weight measured in grams.

Time (minutes)	Alginate Gel (0%) - initial mass	Alginate Gel (0%) - final mass	Sand + alginate (30%) - initial mass	Sand + alginate (30%) - final mass
2	7.2263	7.2771	7.1635	7.1985
5	7.1541	7.2629	7.2451	7.2798
10	7.1272	7.1893	7.2635	7.3122
15	7.2149	7.2689	7.2608	7.3152
20	7.2552	7.3022	7.0739	7.1381
60	7.1463	7.2284	7.1481	7.2616

Figure 3. Resulting graph for 30wt% earth sand gel

Figure 4. Resulting graph for 0wt% sand, pure 3wt% alginate

Observations

the alginate gel did not stick to the sides of the vials so calcium chloride solution was diffusing on all sides (not vertical diffusion)
gels are expanding with time and can see slight pink on the surfaces on the gels with sand
can see a pink gel after 5 minutes of crosslinking (none for just 2 minutes)

Summary

Seeing a trend of increased volume absorbed (and colour) with increasing crosslinking time

5mins may be an outlier for alginate (0% sand), will repeat
can also calculate mass of CaCl2 absorbed

Hand off

4mL left of 30wt% 425 µm earth sand, 9wt% CMC and 3wt% alginate gel in the cold room in the plastic cups
The obtained data will be fitted against our theoretical diffusion model