Soft capsules, hard capsules, and hybrid capsules

Capsule structures are often regarded as mimics of living cells and have attracted attention within bio-related fields. In addition, their use in other practical applications has become important. The demands of practical applications have stimulated recent developments in capsule science and technologies. In this review, we focus on recent developments on the preparation and functionalization of capsule objects. In particular, we will summarize research on micro and nano-sized capsules prepared using supramolecular concepts and related strategies. The examples are classified into three categories. Molecular capsules such as calixarene, cavitands, and dendrimers are introduced together with dynamic molecular capsules at the air-water interface in the first part. In the second category, soft polymer capsules prepared by interfacial condensation, template synthesis, and emulsion synthesis are described and the unique contribution of layer-by-layer assemblies is emphasized. In the final part, various hard capsules composed of gold, carbon, silica, and other inorganic substances are described together with hybrid capsules made from inorganic components and organic moieties. In this section, construction of hierarchic structures of mesoporous capsules is highlighted.