Conventional oral drug dosage forms are known to provide a prompt release of active ingredients, but one cannot control the release of the medication and cannot maintain a therapeutic concentration at the desired site for a long time [1]. Various factors affect the rate and extent to which the drug reaches systemic circulation after administration, including the excipients used, physicochemical properties of the active ingredient, physiology and pH of the gastrointestinal tract, gastric emptying rate, and GI motility [2,3]. As the drug concentration in plasma varies with a conventional dosage form, it is challenging to obtain a steady-state drug concentration in plasma. These fluctuating drug levels may hamper the achieving and maintaining of an effective concentration, which may result in an undesirable drug response or may not produce a therapeutic response [4].

To overcome these limitations with conventional dosage forms, controlled-release drug delivery systems, through which the drug is released in a predicted and sustained manner, have been developed. They maintain the drug concentration between the minimum effective concentration (MEC) and maximum therapeutic concentration (MTC) for a prolonged time, ensuring sustained therapeutic action [1,5,6,7,8]. Among the various controlled-release systems, osmotic drug delivery systems provide substantial advantages. Since these systems maintain uniform plasma drug concentrations and provide a prolonged therapeutic response, they minimize the dosing frequency and subsequently improve patient compliance [3]. Moreover, they release drugs at a rate that is independent of physiological factors, including GI motility, food, pH, and the hydrodynamics of the dissolution medium. This results in comparable release patterns and an excellent in vitro/in vivo correlation. Another superior advantage of osmotic systems is that they are suitable for use with drugs that have a broad range of water solubility and are easy and simple to formulate, operate, and scale up [1,9,10,11].

Osmosis can be defined as the net movement of water across a selectively permeable membrane due to pressure [12]. There is a concentration difference of the solute across the membrane, which is semi-permeable, creating pressure. Water is allowed to pass through this membrane, and solute molecules are mostly not allowed to as this membrane is selectively permeable. The pressure applied to the higher-solute-concentration side to resist solvent flow is called the osmotic pressure [13]. Osmosis can also be defined as the movement of water from a region with a higher concentration to a region with a lower concentration across a semi-permeable membrane [14].

Osmotic delivery systems or osmotic pumps are mainly composed of a core containing a drug and an osmogen. These are coated with a semi-permeable membrane containing one or more ports for drug delivery, such that the drug is released over time in the form of a solution or suspension [15]. Oral osmotic systems are composed of a compressed tablet core coated with a semi-permeable membrane through which delivery orifices are created using a laser beam or mechanical drill [10]. These controlled systems are based on osmosis and osmotic pressure and are independent of various gastrointestinal factors. However, it is noteworthy that there are critical factors that influence the design of osmotically controlled drug delivery systems, including the drug solubility, delivery orifices, osmotic pressure, semi-permeable membrane, type and nature of the polymer, membrane thickness, and type and amount of plasticizer [16,17].

Advancements in drug delivery technology from the oral route to a specific target have been made over many decades [18]. Besides the oral route, the vascular route of administration has also been widely used. Drugs administered intravenously benefit from avoiding the acidic and enzymatic gastric environment. However, the drawback of the intravenous route is its invasiveness [19]. The place where the catheter or needle is to be applied also requires proper cleanliness, as there is a risk of infection when the therapy is continued for several weeks [20].

The implanted pump was the next advancement—a device designed for implantation under the skin to bypass the application of the catheter. These devices provide constant drug release [21]. A brief description of its advantages and disadvantages is summarized in Table 1. Further advancements in implantable pumps occurred in the 1970s, at which point they were employed in animals only. Osmosis was the principle of this new approach, and these new pumps could be smaller than other constant-rate pumps [22]. Alza Corporation and Felix Theseus made several alterations and improvements to this concept in the 1970s and 1980s, resulting in the construction of the elementary osmotic pump (EOP) [20].

Osmotic delivery systems were first studied in 1748, and a further achievement came in 1877 when osmotic pressure was measured quantitatively. In 1955, the first implantable osmotic pump was developed by Australian pharmacologists, which was named the Rose–Nelson osmotic pump. In 1973, several modifications were made to this pump by Higuchi and Leeper, who introduced the Higuchi–Leeper osmotic pump [23]. Several patents have been granted for osmotic pump system, suggesting that there is a great demand for these products. Oral osmotic pumps are also called gastrointestinal therapeutic systems, such as the EOP [9]. A patent was granted to Alza Corporation in 1976 for an oral osmotic pump. The osmotic bursting drug delivery system was then developed in 1979 [24].

This system was modified in 1982 by adding a hydrogel layer that has the property of swelling in the presence of fluid. A patent for this was granted [25]. Push–pull osmotic pumps utilized in combination therapy were first introduced in 1984 [26,27]. One year later, in 1985, mechanically drilled orifices were removed, and the resulting product became known as the controlled-porosity osmotic pump system; this was patented the following year [28]. It was then possible to use liquid pharmaceutical agents in osmotic systems, and this type of system was granted a patent in 1995. In these systems, the agents are enclosed in a capsule with a delivery port and a layer of osmogen. A semi-permeable membrane surrounds this [29]. Osmotic-pressure-dependent drug delivery via a capsule with an asymmetric membrane was introduced in 1999 [12].

Not every administered drug needs to provide a prolonged response, so the osmotic pump system is not suitable for all drugs. Drugs that are indicated for the prolonged treatment of diseases with a biological half-life in the range of 1–6 h are best suited for osmotic systems. Drugs with a biological half-life shorter than 1 h are not good candidates, and, similarly, drugs with a half-life greater than 12 h are also not good candidates for controlled release in an osmotic pump system. The drug’s half-life should be short so that it can be sustained or maintained in plasma, and its prolonged release should be the requirement [38]. To be incorporated into this system, drugs should also be neither highly soluble nor very poorly soluble, and the nature of the drug should be potent for this purpose [39,40].

Osmogens and osmogents are other names for osmotic agents, and they create the osmotic pressure in the osmotic delivery system. When a drug has low solubility, it will be released at a slow, first-order rate; to make this release rate faster, osmotic agents are used in the formulation. These agents generate a high gradient of osmotic pressure within the osmotic system; thus, the rate of drug release increases [41]. The osmotic agents available on the market include lactose, fructose, sorbitol, dextrose, sodium chloride, citric acid, potassium chloride, sucrose, xylitol, and mannitol. Osmogens may also consist of mixtures, such as mannitol + sucrose, dextrose + fructose, sucrose + fructose, dextrose + sucrose, mannitol + fructose, lactose + fructose, mannitol + dextrose, or lactose + dextrose [42]. Drugs with good water solubility can be used as osmotic agents, such as mannitol, glycerol, lactulose, sorbitol, or polyethylene glycol. However, osmogenic salts (e.g., sodium chloride and potassium chloride) and sugars can be incorporated into the formulation if the drug itself does not possess osmogenic activity. Hence, water solubility and osmotic activity are the two most important determinant factors when selecting osmotic agents [43].

As the nature of the osmotic system membrane is selectively permeable, the polymer should be selectively permeable, to allow for the passage of water only, and should be impermeable to solutes [44]. In osmotic pump preparation, the polymer that is most commonly and extensively used is cellulose acetate, which is provided in various grades of acetyl content [45]. The grades containing 32% and 38% acetyl content are most commonly employed. The degree of substitution (average no. of hydroxyl groups replaced by substituting groups) determines the acetyl content. Other digestive polymers used for this purpose include cellulose esters, like diacetate, propionate, cellulose acetate, triacetate, and cellulose acetate butyrate. Ethers of cellulose can also be included in this, such as ethyl cellulose [46,47]. The material must have sufficient wet strength to retain the integrity of its dimensions, which is beneficial for the device. The ability of the material to allow water permeation must be sufficient so that the flux rate of water stays within the required range. The transmission rates of water vapor can be calculated to estimate the water flux rates. The biocompatibility of the membrane material should also be considered [48]. Common biocompatible polymers include PEG, HPMA, PGA, chitosan, and dextran. These materials, when used in oral systems, can be ingested and then excreted in feces once the osmotic pump is exhausted. Oral and implant fractions that may have been absorbed are very likely to be eliminated by glomerular filtration in the kidney, provided that they are below the glomerular threshold [49].

Wicking agents are substances with the ability to absorb water into the porous network of a delivery system. Their main function is to carry solvent molecules to surfaces inside the core of the osmotic device, thereby creating channels of enhanced surface area. A wicking agent is selected based on its nature, either swellable or non-swellable, and based on its ability to undergo physisorption with solvent molecules. Physisorption is a form of van der Waals interaction where the solvent molecules can loosely adhere to the surface of the wicking agent [50]. Sodium lauryl sulfate, polyvinylpyrrolidone, and colloidal silicon dioxide are examples of such agents [51].

A microporous membrane forms due to the presence of pore-forming agents. A pore former can also form walls with micro-sized pores. This pore former leaches out, creating pores as the system operates [52]. Alkaline metal salts such as potassium chloride, sodium chloride, and others may be used as pore-forming agents. Alkaline earth metals such as calcium nitrate and carbohydrates such as fructose and glucose can also be employed for this purpose [53].

The solvent system conveys the polymer, which is dispersed or dissolved, and other additives to the substrate surface as its primary function. Solvents that are inert and either organic or inorganic in nature are employed to prepare a polymeric solution [54]. These solvents should not cause adverse actions in the core or other materials. Examples of such solvents include methanol, cyclohexane, methylene chloride, isopropyl alcohol, and water [55].