Polymeric capsules

The formation of polymeric capsules can also be achieved by the cross-linking of self-assembled amphiphilic block copolymers [85]. The hydrophobic section of the polymer in an aqueous solution will tend to aggregate on the interior of the micelle, whereas the hydrophilic ends will form the outer shell of the micelle. If the hydrophilic end is appropriately functionalized, it can be cross-linked, giving a polymeric shell. The overarching concept is shown in Figure 5.10. 

R. K. Castellano, D. M. Rudkevich, J. Rebek, Jr, Polycaps reversibly formed polymeric capsules , Proc Natl Acad. Sci USA 1997, 94, 7132-7137. 

Castellano RK, Clark R, Craig SL, Nuckolls C, Rebek J Jr. Emergent mechanical properties of self-assembled polymeric capsules. Proc Natl Acad Sci USA 2000 97 12418-12421. 

Defining inside from outside is a fundamental trait of living organisms. The creation of noimamral strucmres that can define in from out with nonpermeable or semipermeable barriers offers the potential of protecting the internal content from destmction, contamination, and unwanted dispersal until the content is dehvered to a defined location. Small spherical structures that define in and out are well known and come in forms ranging from microcapsules to vesicles to micelles. We refer to these structures collectively as polymeric capsules. 

Synthesis of polymeric capsules involves polymer self-assembly on many levels and length scales. Approaches to their synthesis include cases where monomers are polymerized around a liquid template, premade polymers are assembled around liquid and solid templates, premade particles are assembled around templates, and small molecules or polymers assemble spontaneously. Each method offers unique challenges and advantages, and each allows the production of capsules with different properties. 

Being recent research arrivals to the field ourselves, we wrote this chapter in a style that we hope will entice others to enter it as well. We begin with a brief overview of methods used to make capsules, making sure to reference other, more comprehensive sources in each case. We also survey the catalytic capsule field and capsules as drug-delivery vehicles, highlighting recent innovations in both areas. We hope this chapter will convince you that polymeric capsules are exciting materials with equally exciting applications. 

Hollow polymeric capsules are usually produced using one of three methods (Fig. 8.1). 

Polymeric capsule preparation is achieved by self-assembly of small molecules, polymers, and particles into nanometer- and micron-sized objects. Despite the age of the field and number of materials produced thus far, researchers have only just begun to tap the full potential of polymeric capsules that are partially illustrated here with examples of catalytic capsules and drug-delivery vehicles. 

Figoli et al. [96,97] reported the preparation of polymeric capsules combining the phase-inversion technique with the membrane process. Polyetheretherketone (PEEKWC) capsules of different size (300-800 micrometer) and morphology (asymmetric with a porous or dense layer) have been prepared. The SEM pictures of the prepared PEEKWC capsules are shown in Figure 21.16. The capsules can find application both in chemical and in food packaging fields [98],... 

Bioactive films made by the LbL technique have been extensively studied by many scientific groups worldwide. The films can host not only bioactive molecules introduced as constituents of the film, but also carriers with encapsulated biomolecules, for instance liposomes (Lip) and polymeric capsules (Fig. 2). Stimuli-sensitive... 

Demchuk lA, Nagurskii OA, and Gumnitskii YM. Mass transfer from a sohd spherical particle coated with an insoluble polymeric capsule. Theor. Found. Chem. Eng. 1997 31 339-342. 

Another concept, introduced by van Hest et al (39), used enzymes non-covalently assembled in polymeric capsules forming a metabolic pathway thus, the product of one catalytic step could be directly used as substrate for the next. Lambermon et al (40) developed scaffolds to immobilize enzymes to produce synthetic metabolons. The scaffolds were based on self-assembling organized systems, such as virus particles or aggregates, formed in water by an ammonium terminated surfactant. 

Stefanescu, E. A. Stefanescu, C. Negulescu, 1.1. Biodegradable polymeric capsules obtained via room temperature spray drying preparation and characterization. J. Biomater. Appl., 2011, 25(8), 825-849. 

Fig. 5 Top pairwise selection of guests aithin capsule 5 Bottom reversibly formed polymeric capsules — Rebek s polycaps.

---