The microbiota-brain interaction is a complex network connecting gut microbiota to the enteric nervous system (ENS) and the central nervous system (CNS) that are crucial for neurotransmission and neurological health. Metabolites such as short-chain fatty acids (SCFA), neurotransmitters, and neuromodulators from the gut microbiota influence behavior and brain function. This review focuses on the study related with gut bacteria, including Bifidobacterium infantis and Lactobacillus species, producing various metabolites in gut including bile acids, SCFA, histamine, and others to communicate with the brain or CNS. Dysbiosis can lead to neurological conditions such as anxiety, depression, PD, and AD. SCFAs from gut bacteria bind to the free fatty acid receptors of intestinal epithelial cells (IECs), affecting neurones and influencing neuroactivity. Gut bacteria also produce neurotransmitters that regulate growth hormone release through interactions with the CNS and endocrine systems. Brain signals interact directly with the pituitary and adrenal glands through the hypothalamic-pituitary-adrenal (HPA) axis, which in turn communicates with enteroendocrine cells (EECs). Investigating probiotics, prebiotics, and dietary changes could open new avenues for treatment for mental and neurological problems.

Acyclovir (ACV) controls cutaneous herpes, genital herpes, herpes keratitis, varicella zoster, and chickenpox. From previously reported ACV formulations, we continued to explore the permeation behavior of the optimized ACV loaded optimized ethosome (ETHO2R) and elastic liposome (ELP3R) and their respective carbopol gels across artificial membrane, cultured human EpiDerm, and rat skin. Transepidermal water loss (TEWL), scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), and atomic force microscopy (AFM) were used to investigate the mechanistic perspective of permeation behavior. The size values of reformulated ELP3-R and ETHO2-R were observed as 217 and 128 nm, respectively (close to previous report), whereas their respective gels showed as 231 and 252 nm, respectively. ETHO2R showed high elasticity, %EE, and low vesicle size. These were investigated for the diffusion rate of the drug permeation (3 h) across the artificial membrane, cultured human EpiDerm, and rat skin. ETHO2GR showed the highest permeation flux (78.42 µg/cm²/h), diffusion coefficient (8.24 × 10⁻⁵ cm²/h), and permeation coefficient (0.67 × 10⁻³ cm/h) of ACV across synthetic membrane, whereas diffusion coefficient (2.4 × 10⁻⁴ cm²/h) and permeation coefficient (0.8 × 10⁻³ cm/h) were maximum across EpiDerm for ETHO2GR. ETHO2R suspension showed maximized permeation flux (169.58 µg/cm²/h) and diffusion rate (0.293 mg/cm²/h^1/2), suggesting the rapid internalization of vesicles with cultured skin cells at low viscosity. A similar observation was revealed using rat skin, wherein the permeation flux (182.42 µg/cm²/h), permeation coefficient (0.3 × 10⁻² cm/h), and diffusion rate (0.315 mg/cm²/h^1/2) of ETHO2R were relatively higher than ELP3R and ELP3GR. Relative small size (128 nm), low viscosity, ethanol-mediated ultra-deformability, high drug entrapment (98%), and elasticity (63.2) are associated with ETHO2R to provide remarkable permeation behavior across the three barriers. The value of TEWL for ETHO2R (21.9 g/m²h) was 3.71 times higher than untreated control (5.9 g/m²h), indicating ethanol-mediated maximized surficial skin lipid perturbation at 3 h of application, whereas the respective ETHO2GR-treated rat skin had TEWL value (18.6 g/m²h) slightly lower than ETHO2R due to gel-based hydration into the skin. SEL, CLSM, and AFM provided a mechanistic perspective of ETHO2R and ELP3R-mediated permeation across rat skin and carrier-mediated visualization (skin–vesicle interaction). AFM provided detailed nanoscale surface roughness topographical parameters of treated and untreated rat skin as supportive data to SEM and CLSM. Thus, ethosomes ETHO2R and respective gel assisted maximum permeation of ACV across rat skin and cultured human EpiDerm to control cutaneous herpes infection and herpes keratitis.

Polymer hydrogel used as computer-aided, non-biological arsenal utilize as a drug delivery vehicle overthe past few years.New advances in three-dimensional (3D) bioprinting technology have created new opportunitiesfor the use of hydrogel polymer-based medication delivery systems. 3D printing can deliver the ideal shapes or changecapabilities under specific circumstances which have a better adaptation to physiological function. The accuracy of 3Dprinting technology was significantly higher than that of conventional production techniques.A model bioink acquireproper physicochemical characteristics (mechanical and rheological) and biological properties important for proper functioning.It acts as additive manufacturing with complex spatial structure in biomedical research. In this review, we outlined the currentdevelopments in 3D printed polymer hydrogels as delivery and other platforms.