Functionalized nanoparticles and nanocapsules are of high interest in biomedical applications since they can be specifically directed to certain cells. The miniemulsion technique is a convenient and powerful tool for the preparation of the specified polymeric particles or capsules with sizes ranging from 150 to 300 nm.

The basic step for the particle preparation is the formation of a stable monomer-in-water miniemulsion. The particles are subsequently formed by polymerization of the respective monomer. As an example, biodegradable poly(n-butylcyanoacrylate) nanoparticles and nanocapules were prepared by anionic polymerization after the addition of a nucleophile to the miniemulsion. The choice of nucleophile determines the functionalization of the particle surface. The particles can excellently be used to pass the blood-brain barrier; various substances can be encapsulated in the particles and can be delivered in the brain. Biocompatible polyisoprene and polystyrene particles were synthesized through a radical polymerization process. A surface functionalization of the polystyrene particles was obtained by copolymerization with functional hydrophilic monomer such as acrylic acid or aminoethyl methacrylate. The degree of functionalization has a direct influence on the cell uptake of these particles: the higher the functionalization is, the better the uptake in various cell line is. A specific functionalization can be obtained by binding peptides, proteins, or antibodies to the particles' surface. Biodegradable poly(L-lactide) particles have been obtained from the preformed polymer by emulsification/solvent evaporation method, which is based on the precipitation of the polymer within the miniemulsion droplets and formation of the solid particles. For later detection of the particles, a hydrophobic fluorescent dye or magnetic particles was encapsulated into the polymer matrix.

The formation of stable hydrophilic monomer droplets in the continuous organic phase offers the opportunity to perform the reaction not only within the droplets, but also at their interface via interfacial polyaddition. The resulting particles have a "core-shell" structure, consisting of a liquid cavity surrounded by a polymeric membrane. Therefore such capsules give the possibility to protect the encapsulated material from the influence of the environment. The nanocages with the hydrophilic core can be used as a transporter for DNA or drugs. By variation of several reaction parameters such as monomer and surfactant concentration, or the amount of the dispersed phase, the shell thickness can be adjusted to be between 13 and 25 nm. Additionally, a cell marker as for magnetic resonance imaging, fluorescent detection, and x-ray analysis can be incorporated.