Spurred by recent progress in materials chemistry and drug delivery, stimuli-responsive devices that deliver a drug in spatial-, temporal- and dosage-controlled fashions have become possible. Implementation of such devices requires the use of biocompatible materials that are susceptible to a specific physical incitement or that, in response to a specific stimulus, undergo a protonation, a hydrolytic cleavage or a (supra)molecular conformational change. In this Review, we discuss recent advances in the design of nanoscale stimuli-responsive systems that are able to control drug biodistribution in response to specific stimuli, either exogenous (variations in temperature, magnetic field, ultrasound intensity, light or electric pulses) or endogenous (changes in pH, enzyme concentration or redox gradients).

Stimuli-responsive nanocarriers for drug delivery

Chitosan—A versatile semi-synthetic polymer in biomedical applications

Chitosan-based hydrogels for controlled, localized drug delivery

Chitosan-based biomaterials for tissue engineering

In medicine, nanotechnology has sparked a rapidly growing interest as it promises to solve a number of issues associated with conventional therapeutic agents, including their poor water solubility (at least, for most anticancer drugs), lack of targeting capability, nonspecific distribution, systemic toxicity, and low therapeutic index. Over the past several decades, remarkable progress has been made in the development and application of engineered nanoparticles to treat cancer more effectively. For example, therapeutic agents have been integrated with nanoparticles engineered with optimal sizes, shapes, and surface properties to increase their solubility, prolong their circulation half-life, improve their biodistribution, and reduce their immunogenicity. Nanoparticles and their payloads have also been favorably delivered into tumors by taking advantage of the pathophysiological conditions, such as the enhanced permeability and retention effect, and the spatial variations in the pH value. Additionally, targeting ligands (e.g., small organic molecules, peptides, antibodies, and nucleic acids) have been added to the surface of nanoparticles to specifically target cancerous cells through selective binding to the receptors overexpressed on their surface. Furthermore, it has been demonstrated that multiple types of therapeutic drugs and/or diagnostic agents (e.g., contrast agents) could be delivered through the same carrier to enable combination therapy with a potential to overcome multidrug resistance, and real-time readout on the treatment efficacy. It is anticipated that precisely engineered nanoparticles will emerge as the next-generation platform for cancer therapy and many other biomedical applications.

Engineered Nanoparticles for Drug Delivery in Cancer Therapy

The therapeutic effectiveness of chemotherapy is optimal only when tumor cells are subjected to a maximum drug exposure. To increase the intratumoral drug concentration and thus the efficacy of chemotherapy, a thermoresponsive bubble-generating liposomal system is proposed for triggering localized extracellular drug delivery. The key component of this liposomal formulation is the encapsulated ammonium bicarbonate (ABC), which is used to create the transmembrane gradient needed for a highly efficient encapsulation of doxorubicin (DOX). At an elevated temperature (42 °C), decomposition of ABC generates CO(2) bubbles, creating permeable defects in the lipid bilayer that rapidly release DOX and instantly increase the drug concentration locally. Because the generated CO(2) bubbles are hyperechogenic, they also enhance ultrasound imaging. Consequently, this new liposomal system encapsulated with ABC may also provide an ability to monitor a temperature-controlled drug delivery process.

A thermoresponsive bubble-generating liposomal system for triggering localized extracellular drug delivery.

The study was undertaken to investigate the stability of a biological tissue fixed with a naturally occurring crosslinking agent (genipin) at distinct elapsed storage durations. The glutaraldehyde-fixed counterpart was used as a control. Porcine pericardia procured from a slaughterhouse were used as raw materials. After fixation, the fixed tissues were sterilized in a graded series of ethanol solutions and thoroughly rinsed in phosphate buffered saline for 1 day, and then stored in a jar containing sterilized water. The samples were taken out and tested for their stability during the durations of 1day through 6 months after storage. The stability of each study group was tested by measuring its tensile strength, free-amino-group content, and denaturation temperature. Additionally, the cytotoxicity of each test sample and its corresponding storage solution were investigated in vitro using 3T3 fibroblasts. The results were examined using a microscope and 3-(4,5-dimethylthiazol-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. It was found that the stability of the genipin-fixed tissue during storage was superior to its glutaraldehyde-fixed counterpart. The differences in stability between the genipin- and glutaraldehyde-fixed tissues during storage may be caused by their differences in crosslinking structure. There was no apparent cytotoxicity for both the genipin-fixed tissue and its corresponding storage solution throughout the entire course of the study, whereas significant cytotoxicity was observed for both the glutaraldehyde-fixed tissue and its storage solution. However, the cytotoxicity of the glutaraldehyde-fixed tissue decreased with increasing elapsed storage duration, whereas that of its corresponding storage solution increased. This suggested that the toxic residues remaining in the glutaraldehyde-fixed tissue leached out slowly into its corresponding storage solution during the course of storage.

Stability of a biological tissue fixed with a naturally occurring crosslinking agent (genipin).

It was suggested in our previous studies that carbodiimide- and genipin-crosslinked gelatin hydrogels could be used as bioadhesives to overcome the cytotoxicity problem associated with formaldehyde-crosslinked gelatin hydrogels. In this study, we investigated the crosslinking structures of carbodiimide- and genipin-crosslinked gelatin hydrogels. We found that crosslinking gelatin hydrogels with carbodiimide or genipin could produce distinct crosslinking structures because of the differences in their crosslinking types. Carbodiimide could form intramolecular crosslinks within a gelatin molecule or short-range intermolecular crosslinks between two adjacent gelatin molecules. On the basis of gel permeation chromatography, we found that the polymerization of genipin molecules could occur under the conditions used in crosslinking gelatin hydrogels via a possible aldol condensation. Therefore, besides intramolecular and short-range intermolecular crosslinks, additional long-range intermolecular crosslinks could be introduced into genipin-crosslinked gelatin hydrogels. Crosslinking a gelatin hydrogel with carbodiimide was more rapid than crosslinking with genipin. Therefore, the gelation time for the carbodiimide-crosslinked gelatin hydrogels was significantly shorter than that of the genipin-crosslinked gelatin hydrogels. However, the cohesive (interconnected) structure of the carbodiimide-crosslinked gelatin hydrogels was readily broken because, unlike the genipin-crosslinked gelatin hydrogels, there were simply intramolecular and short-range intermolecular crosslinks present in the carbodiimide-crosslinked hydrogel. In the cytotoxicity study, the carbodiimide-crosslinked gelatin hydrogels were dissolved into small fragments in the cultural medium within 10 min. In contrast, the genipin-crosslinked gelatin hydrogels remained intact in the medium throughout the entire course of the study. Again, this may be attributed to the differences in their crosslinking structures. The genipin-crosslinked gelatin hydrogels were less cytotoxic than the carbodiimide-crosslinked gelatin hydrogels. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 4017–4026, 2004

Crosslinking structures of gelatin hydrogels crosslinked with genipin or a water‐soluble carbodiimide

The invention discloses the nanoparticles composed of chitosan, poly-glutamic acid, and at least one protein drug or bioactive agent characterized with a positive surface charge and their enhanced permeability for paracellular protein drug and bioactive agent delivery.

Hsiang-Fa Liang

Papers

A thermoresponsive bubble-generating liposomal system for triggering localized extracellular drug delivery.

Stability of a biological tissue fixed with a naturally occurring crosslinking agent (genipin).

Crosslinking structures of gelatin hydrogels crosslinked with genipin or a water‐soluble carbodiimide

Nanoparticles for protein drug delivery

A Liposomal System Capable of Generating CO2 Bubbles to Induce Transient Cavitation, Lysosomal Rupturing, and Cell Necrosis