Pellicle formation

Soft Gelatin Capsules Dissolution (or disintegration, if justified), microbial bioburden, pH, leakage, and pellicle formation. 

Besides taking part in acquired pellicle formation on tooth (denture, implant) surfaces, MUC5B type mucins cover all oral surfaces with a 10-20- im thick layer. In addition, MUC5B type mucins form a hydrophilic viscoelastic gel (already in low concentration) that causes a high viscosity matrix of saliva. 

Nevertheless, the problem of pellicle formation and eventual fall in dissolution rate is still of concern, as the drug bioavailability may be influenced if there is a severe challenge. There is a report where exposure of phenytoin capsules to high humidities resulted in poor dissolution as well as destruction of clinical efficacy. Moreover, the enzyme test is not official in pharmacopoeias other than USP and the products stand a chance of being recalled, if the normal pharmacopoeial dissolution limits are not met. 

The cross-linking reaction is normally slow at room temperatures, and it may take several weeks or even months before a perceptible change is observed. High temperature accelerates pellicle formation through the process of denaturation by increasing the rate of cross-linking reactions and a typical example is... 

It is always desirable to have some sort of preassessment whether dosage forms containing gelatin would show a decrease in dissolution rate due to pellicle formation with time on storage. Sometimes, the formulations are also intentionally subjected to stress conditions to judge the efficacy of enzymes in the dissolution medium for their capacity to overcome pellicle formation. For this purpose, a few approaches have been used successfully. 

Till date, the problem of pelliculization of gelatin products has found major solutions in 1) understanding of the chemistry 2) identification of stabilizers that can be added in formulation fills or films 3) development of rapid test methods for evaluation of possibility of pellicle formation and 4) introduction of two-tier... 

Pellicle formation is determined by adsorption of components from saliva, crevicular fluid and bacteria onto the enamel surface [2-13], Formation of the acquired pellicle is a highly selective process, since only a fraction of the proteins available in saliva is found in the pellicle [14,15],... 

The first stage of pellicle formation is characterised by an almost instantaneous adsorption of salivary proteins on the enamel surface. This initial adsorption process starts within seconds and probably takes a couple of minutes to be completed [13, 16-19], The thickness of this initially adsorbed layer ranges between 10 and20nm [17] (fig. 2). 

The first level of pellicle formation may be explained by the adsorption of discrete proteins onto the enamel surface. In contact with the watery electrolyte... 

Fig. 3. Schematic drawing of pellicle formation indicating adsorption of salivary proteins to the enamel surface via various electrostatic interactions.

The rapid initial phase of salivary protein adsorption is followed by a second, comparatively slower phase of protein adsorption onto the protein-coated enamel surface. The second stage of pellicle formation is characterised by a continuous adsorption of biopolymers from saliva. This process involves protein-protein interactions between already adsorbed proteins, immobilised in the pellicle layer, and proteins as well as protein aggregates from saliva. Amino acid and Auger analyses of the pellicle layer formed on buccally carried enamel slabs [18] indicate that the adsorbed proteins reach an initial thickness in about 2-3 min, and stay at that level for about 30 min. The thickness of the pellicle then increases to about three times its initial thickness and reaches a plateau after 30-90min [5, 18, 27], Within 60min, the thickness of the in situ-formed pellicle will further increase to between 100 and lOOOnm [17, 28], dependent on the supply of locally available salivary biopolymers and the prevailing intraoral conditions [17,28,29] (fig. 2). 

The pellicle layer formed in situ over periods of 30-120 min reveals a knotted, globular surface texture with diameters of the adsorbed globule-like structures varying between 80 and 200nm [37, 38], Such observations also indicate that in vivo pellicle formation is mainly caused by adsorption of protein aggregates rather than by individual salivary biopolymers (fig. 4). 

Several studies have been performed in order to determine the protein composition of the in vivo-formed salivary pellicle, using amino acid analysis and immunological, histochemical, chromatographic and electrophoretic methods [2-5, 7-13, 39-41, 46, 47, 48-52], In general, these studies indicate that proteins and glycoproteins are the major salivary components of the in vivo pellicle. A large number of specific proteins involved in pellicle formation in vivo have been identified by the methods used in the above-mentioned studies and these are summarised in table 1. 

Recently, MALDI-TOF (matrix-assisted laser desorption/ionisation-reflection time-of-flight) mass spectrometry was introduced as a new approach for the investigation of pellicle composition [39], Using mass spectrometry for compositional analysis, it was found that more intact salivary protein species were present in an in vitro-formed pellicle compared to an in vivo-formed pellicle [39], This finding suggests that the in vivo pellicle is an entity formed with components undergoing more extensive enzymatic (proteolytic) processing than in the in vitro pellicle. Therefore, in vitro-formed pellicle layers cannot completely mirror what occurs within the oral cavity [39], This difference may be due to differences in the proteolytic capacity of the saliva supernatant used for in vitro pellicle formation and that of the oral environment. In addition, a particular saliva sample used for in vitro pellicle formation is a closed system, whereas the oral environment is an open system with continuous influx and clearance of oral fluids [39]. 

Regarding the apparent importance of lipids for the protective properties of the pellicle, it should however, be kept in mind that the existing knowledge is solely based on two publications [58,59]. Therefore, additional research is necessary to clarify the physiological role of the lipids involved in pellicle formation. 

The prevailing knowledge on the ultrastructural pattern of pellicle formation and the micromorphological appearance of pellicle is mainly based on (conventional) transmission and scanning electron microscopic investigations [3, 17, 29, 60-67], Only a very few results have been published using novel techniques, such as cryo electron microscopy [68, 69], CLSM [28], or atomic force microscopy [19, 38, 70] for analysis of the pellicle. 

From early TEM studies, matured in vivo-formed pellicle has been described as a homogeneous and amorphous, bacteria-ffee surface coating of varying thickness [62-64, 67] (table 2). Less pronounced pellicle formation has been reported on lingual and labial sites of the teeth, as compared to the proximal areas [62, 63, 67], In self-cleansing sites of the teeth, the pellicle thickness ranges between 30 nm and 80nm [63], whereas in proximal areas the pellicle can be up to 2-pm thick [63],... 

More systematic SEM and TEM investigations of in vivo salivary pellicle formation at various time intervals indicate a more complex ultrastructural pattern of the adsorbed pellicle layer [66, 71-73]. The young 2-hour pellicle has been identified as a 100- to 500-nm thick, uneven and incomplete organic coating of the enamel [6,17, 66, 71]. After aperiod of several hours or days, maturation of the pellicle results in the formation of a more compact layer with granular ultrastructural appearance [63, 71-73]. 

The adsorbed pellicle layer probably consists of a random arrangement of diverse salivary biopolymers which are subject to certain changes dependent upon time. The variable ultrastructure of the (outer) pellicle layer reflects the complex processes of adsorption and desorption, which contribute to pellicle formation. Therefore, it is not possible to exactly describe the rate of pellicle formation or the final pellicle thickness. Nevertheless, from previous studies... 

The adsorption of salivary components on enamel during pellicle formation has been shown to be a specific process that might be influenced by dietary components [13, 44, 65, 135, 136], Some in vivo studies have suggested that differences in pellicle formation and composition are correlated with the dietary habits of the individual [44, 135]. The chemical composition of a... 

Only a few studies have been published with regard to the influence of fluoride treatment on pellicle formation and composition. Contrary results have been reported concerning in vitro salivary protein adsorption on fluoride-treated enamel (table 4). Treatment of pellicle-covered as well as bare enamel surfaces with amine fluoride causes a decrease in surface free energy [184, 185], However, fluoridation of the enamel surface with sodium fluoride does not significantly influence the amino acid composition of the pellicle layer adsorbed in vivo [186], In contrast, stannous fluoride treatment of enamel... 

Vacca Smith AM, Bowen WH In situ studies of pellicle formation on hydroxyapatite discs. Arch Oral Biol 2000 45 277-291. 

Skjorland K, Rykke M, SonjuT Rate of pellicle formation in vivo. Acta Odontol Scand 1995 53 358-362. 

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