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.

In a Na₂CO₃ solution, poly-allylamine hydrochloride (PAH) is broken down into colloidal particles of carbamate ions (R–NHCOO⁻) and amino ions (R–NH3⁺) within a certain pH range. Daiguji et al. (H. Daiguji, E. Matsuoka and S. Muto, Soft Matter, 2010, 6, 1892–1897) reported that when CO₂ microbubbles nucleate in this solution, the colloidal particles of PAH stabilize the microbubbles via adsorption. In this study, we successfully controlled the microcapsule radius by changing the concentration of PAH in the Na₂CO₃ solution. The zeta potential of these PAH microcapsules is positive at pH = 8.5, thus allowing poly-sodium styrene sulfonate (PSS) adsorption on hollow PAH microcapsules and the synthesis of bilayer PAH/PSS. Furthermore, the bilayer PAH/PSS capsule solution was titrated until the pH is adjusted to 7.0. The bilayer microcapsules were stable at pH = 7.0, proving that these bilayer capsules can be potentially employed for medical applications. Nevertheless, the success of bilayer fabrication strongly depends on the mass ratio of PAH and PSS. The decrease in the mass ratio of PSS results in irreversible flocculation. On the basis of the analysis of the zeta potentials and Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectra for hollow microcapsules of both PAH and PAH/PSS, we elucidated the adsorption mechanism of PSS onto hollow PAH microcapsules.

Metallic nanoparticles (NPs) possess unique physicochemical properties that have enabled their engineering for loading drugs, contrast agents, and targeting moieties for cellular and intracellular components, highlighting their emerging role as versatile tools in managing neurological disorders. In therapeutic applications, the surface plasmon resonance characteristics of gold and silver NPs and the responsiveness of magnetic nanoparticles (MNPs) to external magnetic fields facilitate the disruption of protein aggregates and the eradication of cancer cells. For diagnostic purposes, the inherent high electron density of metallic NPs makes them effective contrast agents in imaging technologies. Moreover, these NPs have proven their capability to traverse the blood–brain barrier (BBB) and interact with central nervous system (CNS) components. Despite their extensive scientific exploration and promising applications, metallic NPs have not yet achieved widespread clinical implementation, especially in comparison to polymer‐based NPs. This article presents an in‐depth examination of the physicochemical properties of metallic NPs relevant to neurological applications. It summarizes their roles in diagnosis and therapy, focusing on gold, magnetic, silver, titanium, and cerium NPs. Additionally, this document explains the incorporation of metal NPs in their application and their effect on the human body.

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 μm and 1–2 μm 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.