2. Protection of therapeutics from breakdown in vivo while also maintaining biological activity

be achieved, ultimately leading to the formation of ALZA in 1968 for the commercialization of some technologies [ further led to the development of an early drug delivery system approved by the US Food and Drug Administration (FDA) in 1990, Norplant (now Jadelle), a contraceptive composed of silicone rubber tubes implanted in the forearm that releases levonorgestrel for up to 5 years with pregnancy rates of less than 1% per year

An ideal therapeutic drug is expected to treat or cure a disease without resulting in any side effects [ not been achieved so far. Many chemotherapeutics are found to destroy both cancerous and healthy cells within the vicinity of the target site [ drug directly to diseased cell populations. Polymers have been found to permit the creation of “responsive” materials within the host environment and can be formulated with drugs to control release [ the molecular weight of polymers that can be controlled via mono­mer stoichiometry using controlled polymerization strategies like atom transfer radical polymerization, reversible addition fragmen­tation chain transfer, nitroxide-mediated polymerization, and living ring-opening metathesis polymerization. A bioresponsive material is one that can respond to a specific “trigger” inside or outside of the body. Because the body have unique pathological parameters such as pH gradients, temperatures, enzymes, and small molecules, the creation of materials that will respond to physiological

alterations in both space and time is required. Triggers include chemical, biological, and physical stimuli. The chemical and biological ones are intrinsic to the body, while the physical stimuli are extrinsic to the body and can thus be used to quicken sole drug delivery [

The constituents of the human body such as tissues, fluids, and organelles have varied pH values. Areas like stomach, vagina, and lysosomes display acidic pH (<7.0), ocular surfaces (7.1), blood (7.4), and bile (7.8) [

Hydrolysis-sensitive polymeric materials have also been designed, synthesized, and implemented in vivo for drug delivery purposes. Hydrolysis-prone materials by definition can be degraded by water, a trigger that is ubiquitous in the human body. This degradative process most commonly occurs through the nucleophilic addition of water into an electrophilic functional group on a polymer. Com­monly employed electrophilic functional groups on polymers include esters and anhydrides, each of which have been employed in multiple types of responsive materials [

73]. The Gliadel wafer is

76]. These

polymers include poloxamers, poly(N-alkyl acryl amides), poly (N-vinyl caprolactams), cellulose, xyloglucan, and chitosan. These thermo-responsive polymers can be modified via varying the ratio of monomers, endgroup modifications, and post-polymerization modifications to make them suitable for varying applications [

using chitosan nanopar ticles; and (iii) insulin delivery, among others [ pH-responsive materials

a self-circulating system (e.g., the blood stream or intraperitoneal spaces). Accounting for these factors into a drug– polymer design broadens the project scope and challenges interdis­ciplinary research in order to achieve a unified engineered material. It is also important to note that some magnetic responsive systems have been approved by the FDA [

Light-responsive polymers are used as external drug delivery sys­tems that use noninvasive and painless techniques as drugs are delivered by light UV- and visible-wavelength irradiation stimula­tion. The ease by which drugs can be delivered by light stimulation has been a major motivation for the design of systems to respond to this style of noninvasive trigger. Light stimulation drug delivery has been desirable due to the controlled spatial and temporal release of a therapeutic payload with both UV- and visible-wavelength irradi­ation. This technique provides a remote-activated approach that does not require direct patient contact [

Current challenges

There are polymers that can swell or shrink in response to external stimuli. Changes in porosity can result from leaching of ionic cross­linking molecules, which in turn alters the diffusion pathways for sensing molecules [ changes in porosity occasioned as ionic cross-linking molecules are leached, resulting in alteration of the diffusion pathways for sensing molecules. Alginate, a commonly employed polymer that is isolated from seaweed, is relatively biocompatible. Tuning the spatial and temporal release of encapsulated materials is rather challenging, but has been successfully applied for a variety of applications using alginates. A recent example includes the sustained delivery of vas­cular endothelial growth factor (VEGF) and subsequent analogues from alginate to a localized region within the body. Using an injectable alginate design, the controlled release of VEGF was utilized to promote lymphatic vessel development through improved vascularization [ have the potential to create future generations of materials for the paralleled delivery of therapeutics, regional specific sensing, and secondary responses for noninvasive detection.

Transdermal delivery systems can be divided into three generations of development. As stated in European Pharmacopoeia, transder­mal patches are designed to provide the controlled and sustained release of active substances to the systemic circulation after crossing the skin barrier, mostly by diffusion and resulting in a prolonged and adequately constant absorption rate. The majority of the actual patches are within the first generation.

Matrix- and reservoir-type patches can be divided into hydrogel and non-hydrogel polymeric patches. Hydrogel is a water-swollen and cross-linked polymeric network produced by the simple reaction of one or more monomers. Hydrogels have received high attention in the past 50 years, due to their exceptional promise in wide range of biomedical applications [

The commercially available patches are based on synthetic materials that display greater mechanical resistance and non-degradation over time. These materials can be widely used in different patches design such as drug-in adhesive (DIA) and microneedles patches. The DIA patches consist in the incorporation of a drug and possible additives in pressure sensitive adhesives (PSA) that is limited by a backing film and release liner.

Drug release is greatly conditioned by the physical and chemical properties of the drug. Carrier systems ranging from micro- to nanoscale are able to transport a wide variety of therapeutic mole­cules with enhanced solubility and targeting. Such systems can be incorporated during patch production in order improve drug release, enhancing its permeation through skin layers. Nanovesi­cles, for instance, allow the delivery of the entrapped molecules into or across the skin.

administration through transdermal route has been attracting con­siderable attention due to its numerous advantages, such as less frequent, painless, and flexible dosing, as it also generates less amount of dangerous waste. These factors have been stimulating the research and development of transdermal DDS to be effective.

Another class of innovative biomaterials that are pushing forward pioneering medical approaches are the ones taking the name of smart biomaterials, due to their ability to respond to changes in physiological parameters or external stimuli [ materials are able to modify their physicochemical and mechanical properties as a reaction to biological, chemical, and physical signals, i.e., temperature, humidity, pH, redox potential, enzymatic activity, light, and mechanical stimuli. Among these biomaterials, smart hydrogels are frequently exploited for tissue engineering and drug delivery applications [ be cross-linked also using reversible methods, such as physical cross-linking, thermally induced entanglement, and self-assembly, which may allow for controlling drug release and biodegradation

molecules control different cellular functions (e.g., migration, pro­liferation, differentiation, and synthesis of extracellular matrix com­ponents); furthermore, they suppress the local immune system, inhibit fibrosis and apoptosis, enhance angiogenesis, and stimulate mitosis and differentiation of reparative cells.