Stimuli-responsive capsules

In this chapter we will focus on two emerging, rapidly growing points in the field of microcapsules (i) stimuli-responsive capsules and (ii) composite polyelectrolyte/inorganic capsules. Stimuli-responsive polymers are sensitive to specific changes in their environment and can undergo reversible sharp physical or chemical modifications in response to such changes. They have been widely used for... 

A number of studies of the self-association features of TTF-C4P-based systems are ongoing presently in the Sessler and Jeppesen laboratories. The interested reader is advised to consult the current literature for updates that may arise as the result of these efforts. Based on what has been reported to date, it is clear that functionalized calixpyrroles have a role to play in the design and synthesis of stimulus-responsive supramolecular constructs, including capsules and self-assembled polymers. 

Figure 3 presents an example of islet functioning inside a particular capsule. A typical two-phase perifusion profile is noted,similar in quantitative terms to that of unencapsulated islets. Clearly the ratio of the membrane thickness to capsule diameter is an important parameter, with low membrane capsule ratios providing rapid transfer of nutrients and the exodiffusion of insulin. Contrarily, for thick walled capsules of diameter less then approximately one-half millimeter the perifusion response, as measured by the stimulation index and retardation in insulin response to a glucose stimulus, is slower for encapsulated islets relative to free islets. 

Fig. 5 shows microencapsulated mice islets intended for intraportal (liver) transplantation to achieve clinical normoglycemia. The islet s p-cells produce insulin in response to a blood glucose stimulus providing a therapeutic alternative to daily insulin injections. The capsule size is optimized to permit oxygen diffusion ... 

Considering first the role of the material, if that material were totally inert chemically and unable to react at all with the tissues, and if the device were not able to irritate the tissues in any way, the perturbation to the inflammation/repair sequence is minimal, and the result will be the formation of a zone of fibrous tissue analogous to the scar, but oriented in such a way as to envelope the implant. The classical response to an implant is its encapsulation by soft fibrous tissue. On the other hand, if the material is able to react with the tissues, chemically, mechanically or any other way, it will act as a persistent stimulus to inflammation. While there is nothing inherently harmful about inflammation as a response to injury, persistent inflammation occurring as a response to a persistent injury is less acceptable. At the very least, this results in a continued stimulus to fibrosis such that the capsule is far more extensive and may intervene between the material and tissue it is meant to be in contact with (for example bone in the case of joint prostheses) but perhaps more importantly it can change the immediate tissue environment from one of quiescent fibrosis to that of active chronic inflammation. This is rarely the appropriate response and, as noted above, is likely to generate an even more aggressive environment. 

An early example of a self-assembled calix[4]pyrrole system was reported in 1996 by Sessler and coworkers [79]. Solid state analysis of the calix[4]pyrrole monoacid 38 revealed that it forms a homoditopic complex both in the solid state (cf. Fig. 12.24) and in organic media upon deprotonation. This dimer could be broken (i.e., disassembled) by exposure to a source of fluoride anions. However, it is only in recent years that calix[4]pyrroles have been explored for the creation of capsule-like systems that can be responsive to a specific stimulus [79]. 

---