Gelatin Diffusion Experiment Nanotechnology in Medicine Neil S. Forbes

Background The delivery of nanoscale medicines to cells in the human body requires diffusion through tissues, organs and cell membranes This activity explores the affect of different diffusion rates Understanding molecular diffusion through human tissues is important for designing effective drug delivery systems

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

Experiment Overview The diffusion of the dyes is measured 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

Experimental Setup Gelatin cylinders are formed in Petri dishes. This represents tumor tissue. Food dye is added to the space surrounding the gelatin. This represents the lumen of blood vessels. Each day, two images are of dye diffusion are acquired. This captures the penetration of nanoparticles or drugs in to the tumor. The rate of diffusion is calculated from the images. The diffusion rates of different dyes can be compared.

Questions to consider What did you expect to happen? Which dyes do you expect to penetrate better? Does fast diffusion mean greater or poorer retention? Why does diffusion matter? Does retention matter? Could diffusion and retention be optimized?

Start 3 hours Diffusion is first visible 8 hours Green Food Color

12 hours 48 hours 24 hours 60 hours 36 hours 72 hours

Final

Image Analysis Display the images on a computer screen. Distances in the images need to be calibrated. Use a ruler to measure the depth of penetration and the width of the gel on the screen.

Image Analysis II Calculate the absolute penetration depth We know that the gel is 60mm wide The penetration distance is:

Image Analysis III Plot the distance vs. time

Image Analysis IV The linear diffusion rate is the slope of the line: 0.16 mm/hr or 3.8 mm/day

Comparison to Theory Precise image analysis can quantify the dye concentration as a function of position and time This analysis fit well with theoretical predictions

Try image analysis yourself!

Questions to Consider Are the results expected? Which dyes penetrated better? Do your results make sense? Which would penetrate the best? Which would have the best retention? Which would be the best drug (based on transport alone)? Do you have enough information to answer these questions? What else would you need to know? How could nanotechnology be used to optimize drug diffusion and retention?

Relation to Therapy

Results Diffusion is very slow (millimeters per hour) The physical properties of a dye (or drug) affect the diffusion rate

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