Particle Surface Design using an All‐Dry Encapsulation Method

As particle dimensions decrease, surface composition and surface area of particles become leading factors in determining end-use viability. Effective ways to encapsulate and functionalize the surfaces of fine particles become more important as the uses of such particles become more apparent. Carbon nanotubes show improved dispersion and bonding with a polymer matrix after surface modification, making them potential strengthening fillers in light-weight polymer composites. Drug particles and drug-loaded microspheres encapsulated with pH-sensitive polymers provide targeted release based on the pH of the intended environment. Biodegradable poly(alkyl cyanoacrylate) nanoparticles loaded with therapeutic agents and coated with polysorbate enable drug delivery to the brain. Magnetic nanoparticles coated with dextran give enhanced sensitivity in magnetic resonance imaging. Semiconductor nanocrystals coated with amphiphilic polymer shells and other biological interfaces offer the potential for single quantum-dot tracking and sensing in cell biology. These are just a few examples where microand nanoparticles find themselves useful among a wide range of applications in biotechnology, pharmaceutics, optics, electronics, aviation, and aerospace. It is apparent that the surface properties of the particles play a crucial role in determining the overall function and performance of a particle-based device, and in many cases functional polymers are used to define the final nature of the particle surface. This necessitates good strategies for optimal encapsulation, functionalization, and modification of the surfaces of microand nanoparticles with polymeric materials. Current strategies for encapsulating particles with polymers are mainly liquid-based protocols that rely on applying a polymer coating solution onto a particle surface with subsequent removal of solvent. For example, spray coating of a polymer solution onto a fluidized bed of particles allows a polymer coating to form around each particle as the solvent evaporates. In another approach, an emulsification–solvent evaporation technique, utilizing a single or double emulsion of wet polymer capsules around core microdroplets enables a polymer to precipitate around each core particle upon solvent drying. Alternatively, a layer-by-layer adsorption of polyelectrolyte multilayers with alternating charge can be performed on particles in a colloidal suspension. Among dry encapsulation methods, plasma-enhanced chemical vapor deposition (PECVD) is demonstrated to provide conformal solid coatings with polymer-like compositions under a low-pressure gaseous environment. Using vibration or fluidization to agitate the particles during deposition, coatings have been made on drug microcrystals, ceramic nanoparticles, and carbon nanotubes. Atomic layer deposition is another dry method reported to enable layering around particles down to the nanoscale, albeit with inorganic instead of polymeric materials. We present herein a novel dry polymer-encapsulation process for microand nanoparticles. This method, initiated CVD (iCVD), combines the dry CVD environment with a radical polymerization solution chemistry without the liquid phase. By thermally activating a polymerization initiator in the vapor phase and combining with monomer vapor, a polymerization reaction is induced on the surfaces of the particles when the reactive species are adsorbed. There are three major points of note. First, iCVD intentionally bypasses the use of a liquid solvent phase. This may be a significant advantage as there are indications that liquid-based methods that rely on drying out of a wet polymer solution often suffer from particle agglomeration as a result of strong liquid surface-tension forces and increasing polymer viscosity during drying, which creates liquid bridges that bind the particles together especially when particles fall below 100 lm in size. Second, to promote surface adsorption of reactive species, iCVD relies on a cooled particle bed that is at a temperature (usually room temperature) much lower than the initiator activation temperature. This conveniently enables particle materials, for example, pharmaceutical drugs and polymer systems that may degrade at elevated temperatures to be treated via iCVD without compromising their material integrity. Third, by using controlled radical polymerization chemistries, iCVD produces exceptionally clean polymers with stoichiometric compositions, high molecular weights, and with no residual solvents, excipients, glidants, or plasticizers. With plasma-based dry methods, fully functional linear polymers are not produced because the high-energy plasma environment results in nonselective chemistries which lead to crosslinked networks. Pulsing the plasma can allow for greater retention of chemical specificity but complete selectivity is not achieved. Further, the highly reactive plasma and its associated UV discharge C O M M U N IC A TI O N S