Hollow microcapsules are expected to be integral components of drug delivery systems in medical and pharmaceutical applications. Among the methods studied, the bubble template method (Makuta et al. Mater. Lett. 2009, 63, 703-705) should easily fabricate uniform hollow microcapsules covered with biodegradable polymers. In this study, we clarified the two conditions required to fabricate uniform hollow microcapsules using the bubble template method: the stability of uniformly sized microbubbles in a liquid droplet and the release of hollow microcapsules from the droplet. Furthermore, our experiments evaluated the radius distributions of template microbubbles and fabricated hollow poly(lactic acid) microcapsules.

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Hollow capsules are gas-filled spherical particles. Hollow biodegradable capsules with a diameter of a few microns are expected to be used as diagnostic ultrasound contrast agents and carriers of drug-delivery systems, while those with a diameter of a few tens of microns have been widely used in various engineering applications and medical and pharmaceutical applications, providing weight reduction of the material and improved thermal and acoustical insulation. In this study, uniformly sized hollow polylactic acid (PLA) microcapsules with diameters of 10-20 mm and 1-2 mm have been fabricated by two different methods, both based on the nature of microbubbles. One method is based on the growth and/or coalescence of microbubbles inside microdroplets of PLA organic solution dispersed in a continuous aqueous phase, and the other one is based on the spontaneous release of independent microbubbles covered with PLA from millidroplets into the continuous phase. Neither of these methods uses microfluidic devices, and they have thus high potential for mass production without compromising the uniformity.

Due to their particular water absorption capacity, hydrogels are the most widely used scaffolds in biomedical studies to regenerate damaged tissue. Hydrogels can be used in tissue engineering to design scaffolds for three-dimensional cell culture, providing a novel alternative to the traditional two-dimensional cell culture as hydrogels have a three-dimensional biomimetic structure. This material property is crucial in regenerative medicine, especially for the nervous system, since it is a highly complex and delicate structure. Hydrogels can move quickly within the human body without physically disturbing the environment and possess essential biocompatible properties, as well as the ability to form a mimetic scaffold in situ. Therefore, hydrogels are perfect candidates for biomedical applications. Hydrogels represent a potential alternative to regenerating tissue lost after removing a brain tumor and/or brain injuries. This reason presents them as an exciting alternative to highly complex human physiological problems, such as injuries to the central nervous system and neurodegenerative disease.