text: Pullulan bioconjugate systems can be formed to respond to many different stimuli to enhance the release of the drug to the target tissue. These stimuli include pH, temperature, photothermal, electrical, ultrasonic, magnetic, and enzymatic. The pH is often used to target tumor tissues, as the extracellular pH of tumors is more acidic than the normal cells.
  A pullulan and polydopamine hydrogel loaded with crystal violet demonstrated pH responsive behavior due to the protonation of the polydopamine, which increased the release of the crystal violet in the acidic environment. The study showed that at a pH of a normal cell's extracellular environment, 7.4, about 60% of the crystal violet was released compared to the 87% release when in a pH of 5.0. The use of pH responsive systems for the treatment of cancer may aid in the ability to overcome resistance of the drug as well as prevent excess damage to healthy tissue. 
  Another pH responsive pullulan system was formed with pullulan and doxorubicin where the doxorubicin is attached to the pullulan by hydrazone bonds. The drug release of the doxorubicin was tested at two pHs, 7.4 and 5, where the hydrazine is stable at 7.4 and cleaves in acidic environments. The results from this study supported the results from the pullulan and polydopamine study, as doxorubicin was released faster in the acidic environment than the pH that reflected a normal cell's extracellular environment. 
  Temperature can also be used as a trigger to control the drug release from pullulan systems. Thermal responsive pullulan systems can be used in conjunction with thermal generating treatments for cancer in order to improve the treatment. Nanoparticles composed of periodate oxidized carboxymethyl pullulan crosslinked with two Jeffamines were synthesized and demonstrated that the nanoparticle size could be decreased with increased temperature. The nanoparticles decrease in size with increasing temperature due to the increased temperature promoting the hydrophobic interactions of the structure. Altering the temperature can induce heating or cooling dynamics that are reversible, which allows for unique properties in terms of drug release. Pullulan can be altered with photosensitizers in order to provide a controlled thermal reaction in a target area. Spiropyrane can be added to pullulan in order to act as a photosensitizer.
  Electrical stimuli can be used to alter the delivery of drugs through pullulan constructs. A copolymer polyacrylamide-graft-pullulan was synthesized and used for transdermal delivery of rivastigmine tartarate. In this study, the use of electric stimuli demonstrated the ability to increase the diffusion rate and in a way acted as a controllable switch to control diffusion rate. Pullulan systems can be used to enhance ultrasound imaging, as pullulan-graft-poly(carboxybetaine methacrylate) demonstrated the ability to generate carbon dioxide in response to ultrasound, which enhanced the contrast. Superparamagnetic iron oxide nanoparticles (SPIONs) have been generated which have magnetic properties, which showed to improve uptake and also decrease the cytotoxicity. Enzymes can also be used to trigger drug release mechanisms, such as how esterase has been used to cleave photosensitizers from pullulan in order to increase the photodynamic reaction. 
  As demonstrated in the last example, these stimuli response mechanisms do not have to be independent. They can be used in combinations in order to improve the efficacy of the drug delivery.