J o u r n a l P r e -p r o o f electrostatic repulsion, favoring biopolymer-water interactions and reducing biopolymer-biopolymer interactions. This effect is also responsible for increasing interactions with negative surfaces like lipids, cells, drugs and anionic molecules, as well as mucus conferring mucoadhesive properties [35] .
Interestingly, the CS interaction with anionic polymers can lead to gels formation by complex coacervation [36] . Interaction with smaller polyanions such as sodium tripolyphosphate leads to the formation of bridges (junction zones) between the polymer chains forming gel by ionotropic gelation [37] . These kind of interactions make protonated CS interesting biopolymer for the development of pharmaceutical systems such as micro-and nanoparticles, gels and films [38] .
The ability to form particles applied as GRDDSs is associated with characteristics of the polymeric chain, such as DD and MW, as well as pH value of the dispersion medium. CSs with low DD are more efficiently dispersible at high pH due to their high charge density that favor more intensely interactions with polymers, drugs and cross-linking agents, such as sodium tripolyphosphate. On the other hand, CS with high MW shows greater capacity than low MW to increase the media viscosity even at low concentrations, making the diluted/semi-diluted dispersion concentration regime suitable for particles formation [37] .
The knowledge about structural and biological properties may provide a basis for exploring the potential and versatility of CS-based materials applications as smart drug delivery systems. For instance, changes in CS rheological behavior in response to pH changes can favor the fabrication of suitable GRDDSs [39] . At acidic pH, CS can increase the viscosity in the medium, promoting good adhesion. Beyond that, the modification of CS structure can increase the possibilities of its biological applications [6] as shown in Table 1 .
J o u r n a l P r e -p r o o f Table 1 . Biological applications of distinct and modified CS-based materials.
Materials/Systems Remarks Refs. [ 43] In-vivo (rats) test of antiinflamatorey activity in ulcer model of CS high and low MW.
Anti-ulcerative and wound healing abilities.
LMW CS reduced ulcerative wounds.
HMW CS helped retain the stomachal folds rendering a gastro protective effect, also can stimulates collagen syntesis. [44] Antioxidant CS of MW (0.5-1000
KDa) and DD (50 % -low Reduction of both DD and MW make CS more efficient in removing in vitro reactive oxigen [45] J o u r n a l P r e -p r o o f Table 1 . Biological applications of distinct and modified CS-based materials. [51] J o u r n a l P r e -p r o o f Friedman et al. [56] , demonstrated that CS anti-inflammatory activity is related to the blocking of NK-κB in human type 1 mast cells inhibiting the production of proinflammatory cytokines. This effect can occur due to a downregulation of Ca 2+dependent pathways [57] . In addition, CS antioxidant activity is directly associated with the chemical reactivity of the hydroxyl and amino groups from the C-6 and C-2 carbons, J o u r n a l P r e -p r o o f respectively [58] . Substitutions performed on CS by reducing the DD promotes the exposure of more amino groups in the polymer chain increasing its antioxidant activity.
The biopolymer chain reduction (MW) also exerts a positive effect on the CS antioxidant activity. Smaller biopolymer chains are very rigid, so the intra-chain hydrogen bonds that involve hydroxyl groups and amino acids are less intense than the same interactions in larger chains with more conformational freedom [45] . Considering above mentioned properties, CS is widely used to design colonspecific systems. Unlike when CS is associated with GRDDSs that release the drug in gastric media, in colon-specific systems the dissolution of CS in acidic pH is overcome by association or coating with other polymers. These systems pass through the stomach quickly without significant drug release, protecting the drugs until they reach the intestine. In this organ, CS undergoes the action of enzymes from the intestinal microbiota and from its degradation can release the drug [74] .
Thus, a deep anatomophysiology knowledge of the different portions that compose GIT is essential for the correct performance of GRDDSs.
J o u r n a l P r e -p r o o f CS-based systems that present mucoadhesive, swelling, and acid erosion properties emerge as an alternative to explore different delivery strategies for the gastric environment. The oral route is the most used and convenient because it is a noninvasive way to access the stomach, being a safe administration route that allows selfadministration and consequently, greater acceptance from the patient to the treatment [75] . However, the bioavailability of orally administered drugs may be influenced by several parameters such as gastric emptying and gastro intestinal transit, gastric mucosa status, stomach pH [76] .
The stomach is located in the upper left part of the abdominal cavity below the diaphragm. Stomach size can change according to the amount of distention. After meal, approximately 1500 mL whereas the empty volume range from 25 to 50 mL [77] .
Gastric pH in healthy conditions during fasting and fed state are 1.1 ± 0.15 and 3.6 ± 0.4, respectively. The pH returns to basal level between 2 and 4 h [78] . Anatomically, the stomach is divided in two parts: the proximal stomach, consisting of fundus and body, and the distal stomach, consisting of pylorus. The proximal stomach acts as a food reservoir while the distal stomach has a role in food processing, forming the chymus, which will be released later into the small intestine. The distal stomach also acts as a propelling pump that assists in gastric emptying (GE). The rate of GE is influenced by both food volume and gastric content composition [79, 80] . Table 2 describes the main characteristics of upper GIT. 4. Phase IV lasts from 0 to 5 min and occurs between phases III and I.
After meal, the contraction pattern changes from the fasting state to the fed state.
It is known as digestive motility and comprises continuous contractions as in phase II.
These contractions result in size reduction of the food particles, which are propelled into the suspended pylorus. During the onset of the fed state, MMC is delayed resulting in decreased emptying rate [83] . 
During the planning of GRDDSs using CS, some particularities should be taken into account like length of retention on the drug/dosage form in the stomach, penetration in the gastric mucosal layer, stomach pH and stability of the drug/pharmaceutical system and CS protonation, drug's molecule physicochemical parameters, as well as its solubility. Another important factor is the presence of lipids in the stomach. CS may interact with these molecules and cause changes in the drug release profile or even not release it [84] .
The use of CS to obtain these types of systems has already been shown to be effective in terms of improving the gastric distribution of some drugs. Modi et al. [85] developed mucoadhesive CS nanoparticles (NPs) with ketoconazole and observed a 5hour increase in residence and release time and, consequently, absorption of this drug by the stomach. In other study, Arora et al. [86] designed CS NPs containing alginate and pluronic F-127 for gastric amoxicillin release. The authors observed that the drug was protected from acid degradation, CS NPs showed adherence and mucopenetration releasing amoxicillin to deep regions of gastric mucus, increasing its therapeutic effectiveness. In addition, several research have already proven that CS is a polysaccharide that promotes the improvement of gastric delivery of the most diverse drugs, such as beberine [87] , ranitidine hydrochloride [88] , carvendilol [89] and moxifloxacin hydrochloride [90] .
One of the main challenges that motivate the development of new GRDDSs is associated to the drugs physicochemical characteristics such as narrow absorption windows, local activity in the stomach, instability in the colon or distal small intestine and with low solubility at high pH [91] . The use of GRDDSs can circumvent these limitations mainly when associated with biopolymers as CS. This kind of association leads to the design of promising controlled drug delivery systems [92] .
GRDDSs in contact with gastric medium can release the drug into stomach.
GRDDSs can furnish controlled release profile, more efficient and adapted to different needs, promoting important advances in pharmacotechnical aspects [92] .
Regarding the design of pharmaceutical systems for gastric diseases treatment, 
Bioadhesive systems may be comprised of natural or synthetic polymers that can interact attractively with a biological substrate. When the substrate is the mucus or mucosa, such systems are called mucoadhesive [95] . CS has become a popular component of this type of system because it is able to establish various types of mucus interactions with numerous hydrogen bonds and electrostatic interactions, promoting the dosage form adhesion [96] . The application of this system comes with several advantages: gastric drug release, including increased bioavailability, pharmaceutical dosage form (PDF) residence time in the stomach and reduction of first pass metabolism [89] .
For the adhesion process, the GRDDSs and mucus molecules must interact at the interface. These interactions can occur through ionic and covalent bonds, Van der Waals interactions, hydrogen bonds and hydrophobic interactions [97] . The adhesion is marked by two main steps, contact and consolidation stages, as shown in Fig. 4 .
J o u r n a l P r e -p r o o f Stage I -The initial contact between the PDF and the mucus begins with interpenetration of the chains in the mucus and adhesion to the surface [97] . At this stage, the polymer chains begin to interact with the medium and mucus. In cases where GRDDSs is developed with CS, it is essential that the acidic medium solution acts by protonating and hydrating the CS chains expecting more effective interactions with mucus and PDF.
Stage II -PDF adhesion consolidation occurs because the polymers constituting the system are mostly hydrated, in the case of CS, protonated and, therefore, presenting greater conformational freedom, being able to establish cooperative interactions with the mucus. The main interactions that occur in this stage are the hydrogen bonds, Van der Waals and electrostatic interactions [97] . Due to the high conformational freedom of J o u r n a l P r e -p r o o f the polymer chains and the close contact of the mucus, these chains diffuse through the mucosal layer, deepening and establishing more lasting interactions. Other important contribution to this stage is the suction-type effect promoted using polymers with high affinity for water that causes dehydration of the mucus, promoting the adhesion of the system; this mechanism is explained by the dehydration theory [98] .
Nowadays, the phenomenon of mucoadhesion can be explained in part by six theories that individually or together are not able to exhaust discussions on this topic, since it is a complex phenomenon [99] . For CS-based mucoadhesive systems, the principal theories that explain adhesion are wetting, diffusion, electronic interlocking, adsorption and fracture, which are briefly described below.
This approach is applicable to low viscosity liquid and semi-solid mucoadhesive systems. This theory considers the interfacial tension to estimate the spreadability degree of the system in the mucus. After the spreading phenomenon, the system penetrates into deformations and mucus recesses modifying the surface characteristic, interactions and interfacial tension leading to the adhesion phenomenon [99] .
A method to estimate whether a liquid will have good spreadability and adhesion on a surface is the scattering coefficient (S b ) measure, using the Equation 2 where: energies of interfaces are represented by γ B and γ T -interfacial tension between the system and the mucus, and the interfacial tensions of each of the phases by γ B -surface tension of the system and γ T -surface mucosal tension [97] .
As the contact angle is easily calculated, a relation is established between it and the system-mucus interfacial tension. In this technique, a system with equal or close contact angles of 0º is sought [100] . 
This theory proposes that adhesion is a two-way process (Fig. 5 ) due to the diffusion of the polymer chains into the mucus glycoprotein network, as well as mucus diffusion into mucoadhesive system [100] . It is a process in which the penetration rate depends on the diffusion coefficient, MW, cross-linking density, chain mobility/flexibility and expansion capacity of both [101] . From results based on rheological analysis and spectroscopic techniques, the diffusion process is interpreted as a kinetic process, i.e., time-dependent, so that it is possible to estimate the time required for maximum adhesion between two substrates during interpenetration [102] , as depicted in Equation 3.
J o u r n a l P r e -p r o o f
Where (L) is the depth of interpenetration, and (Db) the diffusion coefficient.
The adhesion strength of a polymer is maximal when the penetration is close to the size of its chain. It is assumed that the depth of interpenetration required to produce an efficient mucoadhesion bond lies in the range 0.2 -0.5 μm [99] .
In this theory, when chains with different electrical charges, solvated or not, approach to form ionic pairs, the mucoadhesive process is established. However, the most accepted approach is the occurrence of bidirectional [103, 104] 
According to this theory, after contact between the interfaces, adhesion occurs as the product of two groups of forces, primary and secondary. The primary ones are ionic and covalent, while the secondary forces are Van der Waal's forces, hydrogen bonding, and hydrophobic forces [106] .
Interface characteristics determine which types of forces will predominate in mucoadhesion. In most systems, adhesion occurs through secondary type forces, which are individually weak, but due to their high number they become globally significant [107] .
J o u r n a l P r e -p r o o f
This theory is substantially different from the others, because it measures the necessary force to separate one material from the other one, correlating it with the intensity of the adhesive forces. Thus, it is assumed that adhesion failure occurs between interfaces, ignoring the fact that failure usually occurs due to the low cohesion within one of the components involved in the adhesion process [97, 108] (Fig. 6) . Larger chains as well as systems with lower cross-linking densities have higher fracture forces,
indicating higher adhesion strength [109] .
The adhesion force is calculated indirectly by equation 4, where: Sm is the adhesion force, Fm is the force required for the detachment of the materials and A is the total area of contact between the interfaces. It is a very suitable approach for solid and semi-solid systems in which there is little penetration of the mucus chains [110] . 
The theoretical approach to be considered when working with mucoadhesive systems will depend on factors that may influence PF adhesion, and it is necessary to take into account some parameters for the development of the system. In this session, we will discuss the main factors to consider when the polymer used is CS.
During the development phase of the mucoadhesive systems it is important to establish the optimum concentration of the polymer used, especially in semi-solid systems [111] . In solid systems, it is observed that, to some extent, the adhesion force 
The crosslinking degree is an important factor to consider when using CS to fabricate mucoadhesive systems, because the main method of obtaining solid systems such as nano and microparticles is ionotropic gelation using sodium tripolyphosphate (TPP) or glutaraldehyde [114] [115] [116] [117] . 
The increase in polymers MW can enhance the adhesiveness of mucoadhesive pharmaceutical systems as well as chains flexibility [118] . Long and flexible polymer chains such as high MW CS are able to enter deeper into mucus and establish more interactions because it presents greater mobility than polymers of very short and excessively enveloped chains. However, the use of polymers with excessively long chains is deleterious to mucoadhesive systems because they lose the ability to diffuse and entangle the mucus. In the development of these systems, it is essential to establish Journal Pre-proof J o u r n a l P r e -p r o o f an optimum size for each polymer, since the shape and flexibility of the polymer chain is also a factor to be considered along with the size [97, 107] . J o u r n a l P r e -p r o o f NPs were characterized.
In vitro release assays were performed and the efficiency and encapsulation capacity determined. coating of the Mg microspheres with a thin TiO 2 layer using atomic layer deposition, after Mg-TiO 2 Janus microparticles were then coated with a PLGA film containing the CLR antibiotic payload.
After the drug-loading step, the microparticles were coated with an outer thin CS layer (thickness ~100 nm).
Micromotors were evaluated for drug carrying capacity.
In vivo: Capacity to retain micromotors in the mouse stomach, anti-H. pylori therapeutic efficacy and toxicity assessment. IPB presented triple mechanism of gastroretention (high density, swelling and gastro-adhesive).
Levodopa release from IPB exhibited a more linear profile [124] Journal 
Considering the Table 3 , CS applicability is vast, and it can integrate different types of gastroretentive systems. Despite the use of this biopolymer as carrier since 1980s [125] , recent advances in nanotechnology favored the development of efficient drug delivery systems such as CS-based micromotors and nanosystems. CS remains contemporaneous and promising biopolymer to develop gastroretentive/mucoadhesive systems with controlled release profile. Although CS gastroretention properties are usually ascribed to its mucoadhesivity, other action mechanisms will be discussed in the next sessions.
Expandable gastroretentive systems were originally designed to treat veterinary pathologies with high dimensions of about 15 x 3 cm (length x diameter) by Laby [126] in order to avoid regurgitation after administration and arrival of the pharmaceutical dosage form in the rumen. Posteriorly, they were also adapted for treatment of humans by Johnson and Rowe [127] . Early, veterinary devices exploited the unfolding mechanism and had a ring-shape or multilayer insoluble polymeric sheet from Griffin and Brewer [128] . On the other hand, the first device for human use was a tablet based on thiolated gelatin which after hydration in the stomach and swelling to a larger size, making it impossible to pass through the pyloric sphincter (mean diameter, 12.8 ± 7 mm) [129] . However, some authors relate that the cutt-off size of pylorus cannot be determined exactly [130] . For example, Timmermans [131] stated that the size of GRDDSs can vary from 7 to 10 mm, while Khosla and Davis [132] noted that sizes up to 15 mm in diameter were not sufficient to provide gastric retention.
According to Klausner et al. [133] , the expandable systems should combine a number of characteristics, of which retention in the stomach is cited after facilitated oral administration, high degradability of the dosage form preferentially in the stomach, no J o u r n a l P r e -p r o o f interference in the gastric motility, prolongation of the shelf-life, among others. These systems, also called plug type systems [134] receive this classification precisely because after administration they undergo a great expansion of their structure, which may be related to both a high swelling or unfolding, blocking their passage through the pylorus.
In this way, the permanence of the system in the stomach is increased until drug release begins with subsequent reduction in size as the system is evacuated through the GIT.
It is important to point out the three basic configurations associated with expandable GRDDSs that make clear the understanding of its gastroretention mechanism. First one, an initial state represented by expandable GRDDSs reduced dimensions that facilitate oral administration, followed by a second state represented by its maximum state of expansion which is responsible for gastroretention, and then, the third and last state of minimal dimensions related to the release and depuration of the drug from the system [101, 133, 135] .
Several mechanisms of gastroretention for expandable GRDDSs have been explored, but it should be mentioned that they all show many advantages and also disadvantages over each other [134] . Two basic mechanisms are responsible for the expansion of structures, which are swelling and unfolding, the former being related to an osmotic effect, and the latter to shape memory.
The main mechanism for swelling and drug release is diffusion through hydrophilic polymers, which can absorb water from the gastric fluid through the appearance of pores in the device surface, creating channels along its entire length, leading to a capillary effect [134] . In unfolding systems, the previously large system (uncompressed) with adequate mechanical properties is folded (compressed) to just open after reaching the stomach when capsules are dissolved in the gastric juice [133] .
Different geometrical shapes have been attributed to unfolding systems, such as tetrahedron, ring or planner membrane [15] .
A recurrent problem that can occur with unfolding systems is the fact that their shape memory is often not sufficient, in addition to being influenced by storage stress, and may lead to performance problems in the stomach [136] . Some strategies have emerged to overcome this problem, mainly by exploiting polymers that do not present plastic deformation, but only elastic deformation.
Other forms of expandable GRDDSs have been reported, such as the use of hydrogels that swell to the significant extent primarily as a function of polymer content, such as those made of polyvinyl pyrrolidone cross-linked with albumin [137] .
Superporous hydrogels (those with pores of a size of hundreds of micrometers) have been also used as expandable GRDDSs. In this case, the high porosity of these systems has been related to a very rapid rate of swelling (generally 1 min and an increase of up to 1000 x initial weight), as well as the ability to absorb a greater volume of liquids, both factors contributing effectively to a fast size-increasing [138] .
Regardless of the mechanism responsible for the expansion, after suffering a large increase in size, these systems must have adequate mechanical properties so that immediate erosion does not occur as a result of loosening the polymer chains together with the intense peristalsis of the stomach. Thus, the extensive expansion for swelling GRDDSs, for example, must be attributed to the presence of physical/chemical crosslinking in the hydrophilic polymer network. These crosslinks prevent the dissolution of the polymer and hence maintain the physical integrity of the dosage form [139] .
Despite the effectiveness of these systems in increasing the residence time in the stomach, they must have some properties to ensure patient safety. Therefore, such J o u r n a l P r e -p r o o f devices should not interfere with gastrointestinal motility, should not irritate the stomach mucosa due to its local retention, the device must have rounded edges, and readministrations should only be performed after the biodegradation of the previous system present in the stomach to avoid accumulation with high doses.
As it is known, alendronate -a drug belonging to the class of bisphosphonatesis the drug of choice for treating osteoporosis, acting mainly through specific inhibition of bone resorption. However, its bioavailability after oral administration is significantly low (< 1 %) due to reduced intestinal absorption and the fact that when administered with food it leads to the formation of complexes that cannot be absorbed [140] . Although the scientific literature brings many studies on the development of gastroretentive systems exploring the high swelling capacity of CS, great care must be taken when classifying such systems as expandable. Since this mechanism acts to avoid the clearance of the pharmaceutical form through of the pylorus, a pronounced swelling is required which promotes a considerable increase in the size of the structures. This means that although many hydrophilic polymers undergo swelling when in contact with gastric fluid, it may not occur to the extent necessary to prevent its clearance from the upper GIT. In these cases, its importance is more related to the mucoadhesiveness and/or flutuation of the gel layer formed by CS swelling, as is the case with works that explore the dual mechanism of gastroretention, for example, floating and swelling [141] . The said gel layer of the swollen CS is also related to the decreased toxicity of some drugs interacting with their cationic groups by the presence of negative charges (alendronate sodium, for example). In another study, Park and Kim [144] developed glycol CS superporous hydrogels using a gas blowing method and glyoxal as the crosslinking agent. The results
showed an increase in mechanical strength accompanied by the decrease in swelling kinetics without loss of water absorption capacity, important properties for the development of efficient gastric retention devices.
Superporous hydrogels of rosiglitazone loaded with CS were synthesized using glyoxal as a crosslinking agent for Gupta et al. [145] . The authors explored CS precisely because it is a polyelectrolyte that gives rise to pH-dependent hydrogels which, when in contact with acidic solutions, show a high swelling due to the presence of positive charges in the network, expanding its volume. Swelling rates were dependent on the extent of crosslinking, so that the higher the amount of crosslinking agent, the lower the swelling ratio. Ionic strength was also another factor that affected swelling, which ranged from 11 % for HCl solution (pH 1.2) with 1M NaCl to 156 % for lowest ionic strength (0.0001 M).
Among the strategies used to increase the gastric residence time, magnetic systems are the only gastroretentive delivery systems based on the attraction between two magnets. This approach contain a small internal magnet present in dosage form, and an external magnet attached to the abdomen over the position of the stomach, which retains the system in the gastric region [101, 139, 146] . The extracorporeal magnet allows the control of gastrointestinal transit of the dosage form for a prolonged period of time [139] .
Ito et al. [147] described the first magnetic system in 1990, performed for application to targeting therapy for esophageal cancer. Magnetic granules containing ultrafine ferrite (γ-Fe 2 O 3 ) were guided to the esophagus with an external magnet for the initial 2 min and almost the entire amount of granules were retained in the region after 2 h of administration [147] .
Although there are successful works, few systems with CS using the magnetic approach have been reported in literature. Magnetic hollow spheres coated with multilayer of CS/PAA developed by Zhang et al. [118] showed a sustained drug release and a strong magnetic response to magnetic field, which should provide the drug targeting to a desired tissue and/or organ through the external magnetic field [148] .
The main advantages of magnetic systems are absolute drug targeting for target organs and tissues, the increase absorption and bioavailability of encapsulated drug, as well as the reduction of the concentration of the drug at non target sites [149] . However, the applicability of these systems depends of an external magnet, which must be positioned with high precision degree to allow drug release in the appropriate place, without compromising patient compliance [101, 139] . Moreover, magnetic technical is expensive and requires specialized manufacturing, correct position degree on the J o u r n a l P r e -p r o o f stomach is very difficult to achieve and the magnets must have relatively constant gradients in order to avoid overdosing of toxic drug [149] .
J o u r n a l P r e -p r o o f
Specifically, the design of controlled drug delivery systems (CDDSs) with efficient action on GIT is still a challenge due to the inability to restrain DDSs in specific regions of the GIT as well as the dependence of the unpredictable gastric emptying time [150] . These drawbacks can be overcome by GRDDSs, showing great potential for improving the bioavailability and site-specific absorption of drugs [150, 151] . Among several GRDDSs approaches, floating drug delivery systems (FDDSs) are of particular interest due to their local active and narrow drugs absorption window for specific area. 
Single unit dosage systems are easier fabricated than multiple unit dosage forms.
However, the application of single unit dosage forms are less effective due to their all or nothing emptying process from stomach generating distinct bioavailability and high drug concentration delivered in others sites causing irritation [79, 154] .
Effervescent systems (or gas-generating systems) can be prepared by using swellable polymers and compounds that can generate gas bubbles [79, 135, 154] [79, 135, 154] . As effervescent compounds sodium bicarbonate, tartaric acid and citric acid are the most commonly chosen [79, 135, 154] . The mechanism of action occurs when the fabricated system (swellable polymer+gas generator compounds+drug) is placed in contact with the fluid gastric leading to CO 2 elimination while the polymer swelling with water. The combination of these actions generate effective density less than gastric fluid increasing the time buoyancy besides to favor the controlled and sitespecific drug delivery as shown in Fig. 7 (20 %, w/w) and F10: HPMC K15M (21.42 %, w/w), Na alginate (7.14 %, w/w), CaCO 3 (20 %, w/w)) exhibited good floating total and lag time, swelling ability, adhesion and sustained drug release rate acting as a promising GRDDS. On the other hand, non-effervescent systems (such as hydrodynamically balanced systems, HBS) present one or more gel-forming or high swellable polymers.
This system is prepared by mixing drug and polymer generally encapsulated by gelatin.
The mechanism comprises fast dissolution of the capsule in gastric fluid followed by swelling of the polymers producing a buoyance gelatin mass with density lower than 1 g cm -3 . The formed gelatinous barrier retains the capsule shape, avoiding the system disintegration promoting controlled drug release. The gelatinous surface erosion favors water permeation into inner layers preserving the surface hydration and the air trapped by the swollen polymers promotes buoyancy to the dosage forms, as depicted in Fig. 8 [ 79, 135, 151] . In addition, drug release is controlled by diffusion and dependent on the J o u r n a l P r e -p r o o f gel barrier erosion/dissolution. If the formulation is composed by fatty or viscous excipients, low-density systems could be achieved, decreasing water penetration and erosion process, which can reduce drug release rates [107, 132] .
Soni et al. [158] The main drawback of single layer HBS consists on the dependence of the air arrested in the dry mass and the characteristics of the swellable polymers [135] .
Considering the above mentioned, some approaches [135, [159] [160] [161] have been developed to improve the efficiency of these dosage forms such as the design of bilayer formulations, as displayed in Fig. 9 . In these dosage forms, one layer promotes the fluctuation in gastric liquid whereas the other one control the drug release [135] . 
Hollow microspheres (or microballoons) comprise low-density polymers with low-density core relating to the shell. Most of them are MUFS fabricated by emulsion solvent diffusion method that entraps oil or air in hollow core [135, 163] . Polymers such as polycarbonate, Eudragit S, cellulose acetate, calcium alginate, agar, CS and low methoxylated pectin are commonly used to fabricate hollow microspheres and foamparticles [168, 169] . Great buoyancy and drug release from these systems depends on polymers amount, the plasticizer polymer ratio and the solvent used for formulations J o u r n a l P r e -p r o o f [135] . Fig. 10 displays hollow (low-density) microalloons (Fig. 10a ) and foam-particles ( Fig. 10b) obtained by solvent evaporation. Pawar et al. [153] formulated microballoons as GRDDS based on mucoadhesive and floating mechanisms to delivery norfloxacin (NFX). These systems were fabricated by using HPMC and ethylcellulose (EC) as core matrix by solvent evaporation method.
CS coating was prepared via ionotropic gelation method to improve mucoadhesion properties and increasing gastroretention time of the microballoons. Good in vitro buoyance was achieved for these systems. Pharmacokinetics results displayed an enhancement of NFX bioavailability for these systems. In vivo assays showed that the microcarriers could prolong half-life of NFX and increasing plasma drug concentration.
Another system that has attracted great attention in the administration of drugs to treat gastrointestinal infections is the raft forming systems. The mechanism of raft preparation consists on the formation of viscous cohesive gel containing entrapped CO 2 gas in contact with gastric fluid [79, 135] . Usually, the formulation containing gel forming agent and antiacids such as alkaline bicarbonates or carbonates to reduce gastric acidity. The buoyance of raft systems occurs by CO 2 formation and elimination, J o u r n a l P r e -p r o o f acting as a blocker to preclude reflux of gastric media [79, 135] . Fig. 11 depicted a schematic illustration of raft forming systems as barrier in specific stomach region. Figure 11 . Schematic illustration of the barrier formed by a raft-forming system.
Raft forming systems usually form a low-density layer on gastric media with high surface area regarding tablets, favoring drug release and the bioavailability enhancement. Beyond that, these GRDDSs show fast buoyance than other floating dosage forms, improving therapeutic efficacy been easily administrated (once a day) to the patient [139] . On the other hand, the stability of these GRDDSs are susceptible to chemical modifications, like: oxidation and hydrolysis. Exposure to the temperature and pH variation as well as types of radiation (ultraviolet-visible, X-ray, among others) could also promotes modification in these systems [139] . El Nabawari [170] demonstrated the preparation of a showing good MbH bioavailability compared to the commercial product. These results suggest the designed raft-forming system as a potential GRDDS to prolong drug action aiming clinical trials [170, 171] .
Different of low-density systems, dosage forms with density next to 3 g mL -1 can sink to the bottom of the stomach to withstand in vivo peristaltic movement remaining intact independently of the GIT disturbance as depicted in Fig. 12 [110] . By using high-density systems (from 2.5 to 3.0 g mL -1 ) the gastric retention time can increase up to 25 h [155] . These systems are formulated with inorganic compounds such as barium sulphate, zinc oxide, iron powder and titanium dioxide increasing the density of the dosage form. However, up to now no effective action of these systems were detected in human just in pre-clinical assays [151, 155, 172] . In addition, the main drawbacks regarding high-density systems consist on the fabrication of pellets with high drug concentration and achieve systems with a density next to 3 g mL -1 [172] .
Journal Pre-proof J o u r n a l P r e -p r o o f Figure 12 . Schematic illustration of the gastroretentive drug delivery system based on high-density dosage form.
CS is a biopolymer with a wide applicability, highlighted as a gastroretentive drug delivery system-forming material, because it brings together special and desirable biochemical properties in this type of system. These characteristics enable the fabrication of CS-based drug delivery systems for the stomach with mucoadhesive, floating, expandable, biocompatible, nontoxic and biodegradable properties. Advances in gastroretentive drug delivery system development technology associated with the versatility of CS will be able to deliver increasingly efficient and selective dosage forms with new mechanisms of gastroretention, as well as capable of increasing the capacity of drug internalization using lower drug doses to reach the therapeutic range, ensuring greater safety.