PH contrast is a technique used to measure pH changes in the extracellular space. This is accomplished by using a gel that has been injected with a solution that contains a chemical that is known to change pH. The gel will then have a colour profile that indicates if the change in pH is causing softening. Gels that have been injected with a pH indicator solution will have a colour profile that shows minimal contrast at the centre of the inclusion, but a brighter appearance around the injection site.
These methods are highly sensitive to variations in pH, and they correlate with the structure and metabolism of tissues. For example, a hydrated hydrogel will have an elastic modulus that increases with increasing pH. However, it is difficult to quantify the impact of pH on the mechanical properties of a hydrogel. Consequently, researchers have been studying the influence of the extracellular pH of hydrogels on their mechanical properties.
To study the effects of pH on a hydrogel’s mechanical properties, gels were injected with a solution of HCl and NaOH. At two hours after injection, a photograph was taken with a digital camera to determine the distribution of pH in the gel. Colour photographs were obtained and analyzed for the average and standard deviation of the colour intensity. Eventually, these measurements were used to estimate pH-induced contrast in gelatin gel phantoms that were imaged with ultrasonic elasticity imaging.
To evaluate the sensitivity of a hydrogel to changes in its pH, a series of homogeneous gelatin gel samples were prepared at pH levels of 4.6, 5.6, and 6.6. They were then tested with classic mechanical techniques. In addition to measuring the mechanical properties of the gel, a series of creep tests were also conducted on the gel. These experiments shared the same experimental geometry as the elasticity imaging studies.
Similarly, a series of ex vivo cartilage samples were harvested from grossly normal tissues. The samples were then soaked in a buffered contrast agent solution for 24 hours. A calibrated pH electrode was then placed in the samples. After thorough mixing, the samples were inserted into a cylindrical container. Several hours later, a pH indicator solution was added.
Using the pH indicator gel, a contrast ratio was determined for each individual pH value. The contrast ratios were then normalized by the values of the pH 5.6 gel samples. As expected, gels whose pH was higher than 6.6 had a higher contrast than those that were lower. The pH e0 profile in the acidic phantom shows that a center of pH approximately 4.6 has been injected, with the surrounding area being more acidic.
The results of this study provide insight into the mechanisms that govern pH-induced contrast in hydrogels. Specifically, they show that the elastic modulus of a hydrogel increases with increasing pH, and that the pH-induced contrast is comparable to variations in collagen density. Furthermore, the effect on mechanical properties is primarily elastic.
Several studies have provided insight into the effects of pH on the lateral contrast ratio profiles of gels. The e0 image is similar to the e1 and e2 images, which are characterized by a larger percent error than e0. Moreover, the lateral contrast ratio profiles suggest that the peak pH from an acid injection is lower than 4.6.