1. Gelatin Diffusion Experiment
Nanotechnology in Medicine
Neil S Forbes

2. Background
The delivery of nanoscale medicines to cells in the human body requires diffusion through tissues, organs and cell membranes
This activity will explore the affect of particle size on diffusion rates
Understanding molecular diffusion through human tissues is important for designing effective drug delivery systems

3. Introduction
Measuring the diffusion of dyes in gelatin illustrates the transport of drugs in the extra-vascular space
Gelatin is a biological polymeric material with similar properties to the connective extracellular matrix in tumor tissue
Dyes are similar in molecular weight and transport properties to chemotherapeutics
Their concentration can be easily determined simply by color intensity
Green food dye contains tartrazine (FD&C yellow #5) and brilliant blue FCF (FD&C blue #1), which have molecular formulae of C16H9N4Na3O9S2 and C37H34N2Na2O9S3, and absorb yellow light at 427nm and blue light at 630nm
Paints contain colored pigment particles that have much higher molecular weights

4. Experiment Overview
The diffusion of the dyes will be compared to the diffusion of paint particles to demonstrate the effect of molecular weight on transport in tumors
Gelatin will be formed into cylindrical shapes in Petri dishes and colored solutions will be added to the outer ring
Over several days the distance that the dyes and particles penetrate into the gelatin cylinders will be measured

5. Experimental Setup
Dissolve gelatin at double strength and heat to dissolve
Lubricate the inside rim of the smaller Petri with holes dilled in the bottom
Invert the small Petri dishes inside each of the larger Petri dishes and inject 10 ml of cooling gelatin
Allow the gelatin to cool for about 20 minutes

6. Experimental Setup-2
Dissolve food dye and tempera paint in water so that the color is strong but still translucent.
Gently and very slowly pull up on the small Petri dish that contains the cooled gelatin. The gelatin will slip off and remain attached to the bottom of the larger Petri dish.
Pour food dye and tempera paint solutions into the region surrounding the gelatin casts (be careful not to get food coloring solution on the top)
Set aside each Petri dish in a level place that will not be disturbed for several days.

7. Analysis
Each day, at 8:30, 12:30, and 4:30 take digital photos or make drawings of the gels
Estimate the distance that the food dye and tempera paint each have penetrated into the gelatin discs
On the last day at the end of the experiment, pour out the food dye and tempera paint solutions
Use a ruler to measure the distance of penetration into the gelatin discs
The rate of diffusion is the penetration length divided by the time
Compare the diffusion rate of the different dyes.
Image analysis will be explained at he end of the experiment

8. Gelatin Diffusion System
A 2-3 mm thick cylinder of gelatin, molded in a large Petri dish
Dyes are added to the space surrounding the gelatin mold

9. Food color vs. tempera paint
Start
3 hours
8 hours
Diffusion is first visible
Green Food Color
Dilute tempera paint

10. 12 hours
24 hours
36 hours

11. 48 hours
60 hours
72 hours

12. Final

13. Questions to consider Are the results expected?
Which dyes penetrated better?
Does that make sense?
Conversely, does fast diffusion mean greater or poorer retention?
How could diffusion and retention be optimized?
Is this the intuitive result?

14. Results Diffusion is very slow (millimeters per hour)
The physical properties of a dye (or drug) affect the diffusion rate
Small molecule food coloring dyes diffuse faster than colloidal suspensions of pigments (tempera)

15. Implications
Understanding the relation between diffusion and convective delivery (through the vasculature) is essential
The properties of delivery systems should be carefully tailored to enhance drug penetration and retention