Controlled drug delivery from hydrogels In the previous module on hydrogel preparation and characterization, you have learnt how to prepare hydrogels and measure some key parameters like ion permeability. Hydrogels are very frequently utilized in various biomedical applications including controlled drug delivery and so this module focuses on designing a hydrogel for control release application. In this module you will learn how drugs can be loaded into hydrogels and how the release behavior can be measured. Finally you will learn how the measured properties can be utilized to design hydrogels for any specific drug release application. Drug release Theory As mentioned before, hydrogels can be adopted to perform a variety of biocompatible tasks including controlled and targeted solute delivery. In order to understand the transport of solutes in gels, a simple solute loading and release experiment can be performed which will yield us the value of diffusivity of solute. Diffusivity gives us an estimate of the hydrogel’s capacity to retain the solute and compare different hydrogels for optimum release rate. In this experiment, after loading a drug (timolol) in the hydrogel, the gel is soaked in pure PBS (buffer) for drug to be released. Under the assumptions that drug transport is diffusion controlled and that perfect sink conditions exist (that the concentration of solutes in the release medium is approximately equals to zero regardless of time), we can write the following equations for 1D Fickian diffusion:

C  2C D t y 2

(1)

C t , y  h  0

(2)

C t , y  0  0 y

(3)

C  y, t  0  Ci

(4)

Where C is drug concentration in gel, D is diffusivity, h = half thickness of gel and C i is the initial concentration of drug in gel. The above set of equations can be solved to:

(5)

Where RD (%) is the percentage of drug released. At short time interval, the equation can be simplified to:

(6) From the slope of a plot of RD% vs sqrt (t), we can estimate diffusivity. UV-Vis spectroscopy Ultraviolet–visible spectroscopy (UV-Vis or UV/Vis) refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible spectral region. This means it uses light in the visible and adjacent (near-UV and near-infrared (NIR)) ranges. The absorption or reflectance in the visible range directly affects the perceived color of the chemicals involved. In this region of the electromagnetic spectrum, molecules undergo electronic transition. Here, we utilize absorbance spectra of timolol to measure its concentration. For low concentrations, the maximum absorbance of timolol (which is at a wavelength of 295 nm) is directly proportional to its concentration. Hence, knowing its maximum absorbance at a given concentration (say at 100 ppm, the absorbance is 2.02), we can find the concentration of timolol in an unknown solution. Measuring the spectra of the release medium at fixed intervals of time will help us estimated the amount of drug released from the hydrogel.

Procedure: A) To be done by previous batch 1 day before the experiment

A1) Prepare 50 ml of 100 ppm timolol solution by mixing 0.005 g of timolol in 50 ml of PBS (buffer) and stirring it 30 min. A2) Drug loading 1. Take 3 clean glass vials with caps and label them 1L,2L and 3L. 2. After the gels have been extracted and dried, take 3 of the gel pieces and weigh them and note the weights. 3. Put 1 gel piece in each glass vial. 4. Add the drug solution. The volume of drug solution in ml = 100 x weight of dry gel in g. Note the volume of solution added. 5. Close the vials. Store them away from direct sunlight.

B) Drug release: Experiment done by current batch B1) Setting the baseline for measuring drug concentration 1. Take a disposable cuvette and put approx. 1.5 – 2 ml of PBS (buffer) using a transfer pipette. Put this inside the cuvette slot in the UV-vis spectrophotometer. Set this as the baseline at a wavelength of 290 nm - 300 under absorption scan mode. 2. After the baseline is set, throw the PBS and the cuvette. In another cuvette, add the original timolol solution and measure the absorbance. Note the absorbance of the loading solution at 295 (absloading = ) B2) Drug release 1. Take 3 clean glass vials with caps and label them 1R, 2R and 3R. 2. From the vials already containing the drug soaked gels, take vial labeled 1L and with a pair of tweezers, remove the gel. Wipe the gel surface very lightly with Kimwipes. Put this gel in vial 1R. Using the pipettes, add the given amount of PBS (buffer). This volume of buffer (ml) corresponds to 100 x wt. of gel (g). Start the stop watch. After removing the gel from vial 1L, close the vial and store it. 3. Do this for the remaining 2 gels sets. But have a gap of 3 minutes between the 3 gel releases. So the 2nd solution is added at t = 3 min and 3rd solution is added at t = 6 min. 4. At t = 15 min, remove approx. 1.5 ml of the solution from the 1 st vial 1R using a transfer pipette and add it to a cuvette. Measure the absorbance at 295 and using the transfer pipette, put it back as much as possible, again using the transfer pipette. Record the measurement in the excel sheet. 5. Repeat the point 4 for vial 2R at t = 20 min and for vial 3R at t = 25 min. 6. Repeat 4 and 5 at t = 30 min, 45 min, 60 min, 75 min and 90 min.

7. Now, measure the absorbance of the drug in the loading solution vials (1L, 2L and 3L)

Record the data and filled in the blanks below.

Time (Min)

Sample 1

Sample 2

Sample 3

15 30 45 60 75 90 Loading solution abs

Put these values in the excel sheet. Follow the instructions given in the excel sheet to find the diffusivity value.

Designing the gels for controlled drug release The first step in designing a gel for any application is characterization of physical and transport properties. After these properties are determined, the next step is to determine the requirements of the specific application and then tailor the gel to meet the design goals. Consider as an example that you want to design a pHEMA based gel for extended release of timolol for 1-week while releasing 0.1 mg of drug each day. You can follow the following approach to achieve your design objectives:

1. Identify the variables that you can manipulate to achieve the design goals. For a pHEMA hydrogel that you have already prepared, the three variables you can control are area, thickness and drug loading. In general you can also control the degree of crosslinking to achieve the desired diffusivity but that might impact other gel properties as well. 2. How does the total release duration from the gel depend on the gel thickness? You can use the formula for % release in Eqs. 5 and 6 to help you answer this question. 3. Since the diffusivity is already measured, you can determine the gel thickness which will give you release duration of 1-week. As a rough guideline you want to choose release duration such that 80% of the drug is released during that time. 4. Since you want to release only 80% of the drug and the desired release is 0.1mg each day for 7 days, what is the total mass of drug that should be loaded in the gel? 5. Assume a gel surface to be circular with 1 cm diameter. Based on this calculate the gel volume and then divide the mass of drug required by the gel volume to determine the desired concentration of drug in the gel. 6. After you have determined the concentration of drug that needs to be loaded, you have to decide how you will load the drug in the gel. You can either load it by direct addition of the drug to the polymerization mixture or by soaking the polymerized gel in drug solutions, as you did in this lab. Both approaches have pros and cons; direct incorporation reduces a processing step but drug can degrade or get irreversibly trapped during polymerization. If loading by soaking in solutions is chosen as the suitable approach, you need to choose the drug concentration in the solution which will give you the desired loading in the gel. Remember that the ratio of drug concentrations in gel and solution is defined as the partition coefficient. You can calculate the partition coefficient of timolol in the gel by following the same procedure as you had used in the previous module to determine the partition coefficient of salt in the gel. After determining the partition coefficient, you can divide the desired drug concentration in the gel by the partition coefficient to determine the concentration of drug in the loading solution.

Note that these calculations assume perfect sink loading so the volume of the solution should be sufficiently large so that the mass of drug in the loading solution is at least 10 times the mass of drug that will be loaded in the gel. Based on this criterion determine the volume of the loading solution. 7. Now that you have designed the gel by deciding the area, thicknesses, and drug concentration, plot the cumulative release profiles as a function of time by using Eq. 5. Also include the desired cumulative release profile based on the desired release rate is 0.1 mg/day. Compare the two profiles and comment on the differences. You will note that the release profile from the gel is such that the release rates are higher than desired at short times and lower than desired at long times. The reason for this is the mechanism of the release, which is controlled by diffusion. The cumulative release profiles increase as sqrt(t) for release from gels, while the desired cumulative release increases linearly with time. These differences could be crucial for some delivery applications, such as drugs with very narrow therapeutic windows.

Post lab exercise

1. Solve the mass transfer problem for release into a perfect sink by using separation of variables to determine the expression for the % release. 2. Solve the mass transfer problem for release into a perfect sink by using similarity solution separation of to determine the expression for the % release valid for short times. 3. How much time is required to achieve equilibrium between concentration in the gel and that in an external fluid? 4. Do a literature search to find some specific applications of control release from hydrogels. 5. For what types of drugs, control release strategy is particularly useful. 6. What is the therapeutic window? 7. What does a zero order release profile mean? 8. Skin patches are commonly used for drug delivery. List some commonly used patches for transdermal drug delivery. Is drug transport from transdermal patches controlled by diffusion in the gel? 9. How can you reduce diffusivity of a drug in the gel?