We have successfully prepared for the first time hydroxyapatite (HAp) and calcium silicate (CaSiO 3 ) nanostructured porous hollow ellipsoidal capsules which are constructed by nanoplate networks using the inorganic CaCO 3 template. CaCO 3 ellipsoids are synthesized via the reaction between Ca(CH 3 COO) 2 and NaHCO 3 in water and ethylene glycol mixed solvent at room temperature and they are used as the Ca 2+ source and cores. Then a PO 4 3À or SiO 3 2À source is added to react with CaCO 3 to form a HAp or CaSiO 3 shell on the surface of CaCO 3 ellipsoids. Dilute acetic acid is used to remove remaining CaCO 3 cores. The size and shape of the HAp and CaSiO 3 hollow capsules are determined by those of the cores. The thickness of the capsule shell can be controlled by adjusting the concentration of PO 43À or SiO 3 2À source. A number of PO 4 3À sources such as dilute H 3 PO 4 , Na 3 PO 4 and Na 2 HPO 4 can be used to form HAp hollow capsules with similar morphologies. The drug loading and release behavior of HAp hollow capsules is also investigated. A typical anti-inflammatory drug, ibuprofen, is used for drug loading. The result indicates that HAp hollow capsules have a high specific surface area and high storage capacity, and favorable drug release behavior.

In many biomedical applications, drugs need to be delivered in response to the pH value in the body. In fact, it is desirable if the drugs can be administered in a controlled manner that precisely matches physiological needs at targeted sites and at predetermined release rates for predefined periods of time. Different organs, tissues, and cellular compartments have different pH values, which makes the pH value a suitable stimulus for controlled drug release. pH-Responsive drug-delivery systems have attracted more and more interest as "smart" drug-delivery systems for overcoming the shortcomings of conventional drug formulations because they are able to deliver drugs in a controlled manner at a specific site and time, which results in high therapeutic efficacy. This focus review is not intended to offer a comprehensive review on the research devoted to pH-responsive drug-delivery systems; instead, it presents some recent progress obtained for pH-responsive drug-delivery systems and future perspectives. There are a large number of publications available on this topic, but only a selection of examples will be discussed.