Stimuli-sensitive, tunable, drug delivery devices have been shown to control drug release over extended periods of time. First generation hydrogels were synthesized purely from artificial materials such as methacrylates. These demonstrate defined volume transitions in response to physicochemical changes in the environment such as pH, temperature, ionic strength, electric field, or concentration gradients of molecules, and respond by releasing drug, protein, or any molecule of biological relevance. The network structure and the thermodynamic nature of the components of these networks play a key role in their diffusional behavior, in their molecular mesh size changes, and in their associated stability of the incorporated biomolecules. At the cutting edge in the field of controlled drug delivery are the second generation hydrogels, crosslinked by recombinant proteins. Such gels, which are conglomerates of biomolecules and artificial building blocks, are termed biohybrid. Three dimensional conformation switches lead to observable volume transitions, which are highly temperature and pH sensitive. Such conformational switches at the molecular level are similar to those found in enzyme catalysis, gene regulation and signal transduction. Unlike proteins, RNA can be programmed and tailored with ease, and also carries genetic information. Unlike DNA, which is purely digital and a carrier of genetic information, RNA is capable of catalysis. The catalytic action is possible due to the unique conformational states adopted by RNA, with hypersensitive responses to ionic, chemical and biomolecular environments. Moreover, small RNAs which regulate genes and developmental timing fall under the immunological radar. This work will produce the third generation of controlled drug delivery devices, by using RNA as the modulating agent, and by taking the level of recognition from the level of the amino acid to that of the nucleotide. Such devices will provide exquisite control of analyte-selective drug release by the programmed swelling states on aptamer-analyte binding.

Ligand-binding RNA pseudoknots were identified using SELEX and affinity chromatography. They were synthesized in vitro using the T7 RNA polymerase and synthetic DNA templates. Transcription of these pseudoknots was controlled by promoters that are known for producing RNA with very high efficiency and yield. Templates and transcripts were examined by agarose and polyacrylamide gel electrophoresis, respectively. Modified nucleotides were co-transcriptionally incorporated into RNA pseudoknots to render them resistant against ribonucleases. Incorporation of fluoropyrimidines allowed the production of RNA resilient against degradation by ubiquitous RNAse A. Protection from other RNases were achieved by posttranscriptional modification of accessible nucleotides with nucleotide specific reagents like kethoxal, DMS, DEPC, etc. Binding properties of RNA aptamers were monitored using modified Sepharoses. A pseudoknot-hairpin-pseudoknot construct was shown to bind and release the ligand by metal ion-chelating agent switching. This new strategy falls under the paradigm of biomimesis, wherein one attempts mimicry of the biological processes, where the molecular recognitive principles are understood, so that they can be exploited and regulated in future treatment regimes.