**1. Introduction**

Since the 1980s, researchers have been working on polymeric drug delivery systems [1–4]. Several of the frontier scientific fields are the hunt for novel medication delivery mechanisms and novel action mechanisms. These include multidisciplinary research techniques that aim to make significant improvements in therapeutic efficacy and bioavailability just at point of medication administration [5, 6]. One or more traditional medication delivery mechanisms are combined with engineering technologies in a drug delivery system. The technologies allow for precise targeting of the place in the body where a medicine has been delivered and/or the pace at which it has been released.

Short half-lives, low bioavailability, and physicochemical instability are all common limitations of biopharmaceutical therapies. Physiological instability is characterized by changes in the highly organized structure of proteins, which can result in undesired events including denaturation, aggregation, and precipitation. The chemical instability of pharmaceuticals is exacerbated by processes including such oxidation, deamidation, hydrolysis, and racemization. Stimulus-responsive polymers provide a pharmaceutical delivery mechanism for delivering pharmaceuticals at a regulated pace and in a durable and physiologically functional state. Research in stimuli-responsive polymers has grown over the years, and a lot of effort has gone into designing eco-friendly macromolecules that may be molded into novel smart polymers [7]. A composition or platform that allows the administration of a medicinal chemical into the body is known as just polymeric drug carriers. By regulating the pace, duration, and location of medication distribution in the system, it increases its effectiveness and safety. There in previous two decades, delivery of drugs has progressed significantly, but regulating medication entrance into the system, particularly the brain, has remained a tough challenge. Recent development in investigations of nano-drug delivery system distribution across the blood-brain membrane via carrier-mediated carriage is starting to give a reasonable basis for directing medication delivery to the brain. Natural materials such as amino acids, hexose, peptides, monocarboxylate, and stem cells are transported over the blood-brain membrane via ingestion transporters [8–10]. In the type of biomaterials with liposomes, polymers in reservoir-containing drug delivery applications have made tremendous development. Additional applications of the polymers include diffusionbased drug delivery systems and solvent-triggered/activated drug delivery systems. Drugs are dissolved in a non-swellable solution or a completely inflated matrix that does not breakdown throughout their engagement period in diffusion-based drug delivery applications. Whenever subjected to an aquatic media, solvent triggered materials such as hydrogels expand and release the medication. They are naturally hydrophilic. Because of their well-engineered polymeric by the changes in the underlying reasons of the biological function, biocompatible polymers provide a safe pathway for medication transport. Biodegradable polymers disintegrate owing to the breakage of covalent bonds among them, whereas bioerodible polymers cause degradation of the polymer owing to the dissolving of connecting strands without causing any changes in the molecule's chemical properties. Aqueous soluble, safe, as well as non-immunogenic polymers are being used as therapeutic carriers. They act in the background to reduce medication breakdown and increase circulation time. Another crucial consideration is the drug's appropriate elimination. If indeed the polymer is non-degradable, it really should be avoided accumulating in the body, and if it is biodegradable, the fragmented elements should be safe and not cause an immune reaction. Polymers that resemble important biological respond to environmental stimuli such as changes in pH or thermal by altering features such as solubility, hydrophobic/hydrophilic equilibrium, biomolecule (pharmaceutical component) releases, as well as configuration [11, 12].

The polymeric medicinal delivery compositions are classified into several classes, such as, biodegradable (chemically-controlled), diffusion-controlled, externallyresponsive systems (e.g., temperature pH,) [13], solvent-actuated [14] and nanosized polymeic delivery platform that accomplish in three prime technologies [15]: (i) PEGylation [16, 17], (ii) active targeting of certain cells and organs [18–20] and (iii) Increased permeability and retaining allows for passive targeting effect [20, 21]. The more sophisticated polymeric therapeutic delivery technologies are indeed being anticipated as multidimensional fully – featured systems that will enable instantly improved pharmacokinetics, decreased toxicity, faster targeting, as well as a
