AquaGel pH

AquaGel®-pH is a superporous hydrogel formulation comprised of a crosslinked copolymer between poly(acrylic acid) and poly(ethylene glycol) which is optimized for environmental sensitivity of the gel to changes in solution pH. Notably, swelling is negligible in low pH (acidic) solutions and drastically increased in high pH (alkaline) solutions. Please note that the provided dry samples are shipped with the poly(acrylic acid) in sodium salt-form and will increase the pH of initial soaking solutions. Once the sodium has washed out, then pH responsiveness will be observed.

Left to right: dry, in pH 1.2 (0.1M HCl), pH 10.6 (NaHCO₃

How it works

The theories which define environmentally responsive hydrogels are the Flory-Rehner and Flory-Huggins theories. Here the polymer solvent-interaction creates osmotic pressure (Δπ_mix) attracting the polymer chains to the solvent which driving the hydrogel to expand. Additionally there is polymer-polymer interactions which counteract the force driving expansion related to the strength of the polymer chains attraction for each other as an elastic force (Δπ_elast). Notably these forces are in a given polymer system are always in balance and as such the following equation applies:

The relative balance of hydrogen protons in an aqueous solution is a stimulus which has a significant capacity to trigger hydrogel responses. This is particularly strong in hydrogels which have ionizable groups (carboxylic acids or amines for example) and hydrogels where ionic bonding plays a key role in their Δπ-bal. In the case of the Aquagel-pH the ionizable groups are carboxylic acids with a pKa ~4.3 and which participate in strong hydrogen bonding with other gel components when protonated but have little bonding when deprotonated. Because of this at high pH the gel swells extensively while at low pH it is reduced. Notably with the Aquagel-pH system this change is reversible and as one switches the polymer from a high to a low pH or vice-versa the gel responds to the change in environment quite rapidly.

What’s it used for?

A wide variety of medical, drug-delivery, tissue engineering, and even industrial applications utilize environmentally sensitive polymers. References below relate to a wide variety of applications of pH sensitive polymers/hydrogels for various uses.

References

Below list is general references to pH sensitive polymers/hydrogels and their uses, not all relate to usage of Aquagel system but rather display the breadth and scope of functional polymer research. This list also includes several handy review articles which provide good background in this technology and its applications.

1. J. Yun, J.S. Im, Y.-S. Lee, S.J. Yoo, H.-I. Kim, Sustained release behavior of pH-responsive poly(vinyl alcohol)/poly(acrylic acid) hydrogels containing activated carbon fibers. Journal of Applied Polymer Science 120(2) (2010) 1050-1056.
2. R.A. Gemeinhart, J. Chen, H. Park, K. Park, pH-sensitivity of fast responsive superporous hydrogels. J Biomater Sci Polym Ed 11(12) (2000) 1371-1380
3. K.M. Gupta, S.R. Barnes, R.A. Tangaro, M.C. Roberts, D.H. Owen, D.F. Katz, P.F. Kiser, "Temperature and pH sensitive hydrogels: An approach towards smart semen-triggered vaginal microbicidal vehicles." Journal of Pharmaceutical Sciences 96(3) (2007) 670-681.
4. J. Kopeck, "Hydrogel biomaterials: a smart future?" Biomaterials 28(34) (2007) 5185
5. C. de las Heras Alarcon, S. Pennadam, C. Alexander, "Stimuli responsive polymers for biomedical applications." Chem. Soc. Rev. 34(3) (2005) 276-285.
6. Y. Qiu, K. Park, "Environment-sensitive hydrogels for drug delivery." Adv Drug Deliv Rev 53(3) (2001) 321-339.
7. Richter, G. Paschew, S. Klatt, J. Lienig, K.-F. Arndt, H.-J.r.P. Adler, "Review on hydrogel-based pH sensors and microsensors." Sensors 8(1) (2008) 561-581.
8. L.-c. Dong, A.S. Hoffman, "A novel approach for preparation of pH-sensitive hydrogels for enteric drug delivery." Journal of Controlled Release 15(2) (1991) 141-152.
9. K. Wang, D. Klimov, Z. Kolber, "Long period grating-based fiber-optic pH sensor for ocean monitoring." Proceedings of SPIE: Fiber Optic Sensors and Applications V (2007) 677019