Biocompatible polymers are one of biomaterials extended a great potential for medical devices and tissue engineering scaffolds. Although, there has been an increase in interest of biodegradable polyesters which are degraded by hydrolysis with or without enzyme. Surprisingly, more than thousands of papers being published in the biomaterials and tissue engineering literature which uses polyester in form of films and scaffolds, there is no in vivo degradation and biocompatible study of polyester in gastrointestinal (GI) tract. In this study, we have performed a method to study the degradable and toxicity of polyester in gastrointestinal (GI) tract.
A number of polymer-based medical devices have been developed for implantation in the human body; these include vascular grafts, stents, scaffold structures, and surgical meshes, among others. Biocompatible and biodegradation polymers, which are new classes of biomaterials, have emerged with high applicability for medical devices, tissue engineering scaffolds, drug delivery, and biomedical-healthcare sensors. In recent years, there has been increasing interest in biodegradable polymers that are degraded through hydrolysis, with or without enzymes, and absorbed in the body via the metabolic pathway.
A majority of the biodegradable polyesters like polylactic acid (PLA), polyglycolic acid (PGA), polycaprolactone (PCL), and their co-polymers, which are used in drug delivery and internal fixation devices, are based on hydrolysable ester bonds; however, the evidence for their biological breakdown in vivo is considerably limited. In biodegradable medical devices, the accumulation of leachable acidic by-products has been found to be a major contributor for the inflammatory response in tissues1,2. In particular, the implantation sites that exhibit low metabolic activity and ability to eliminate by products, may be incompatible for polyester-based materials, which are recently gaining importance. Nevertheless, despite the widely recognized importance of these materials, studying their degradation still poses a significant challenge.
In general, aliphatic polyesters degrade in vivo through random hydrolytic cleavage of ester bonds. The in vivo degradation of polyesters occurs as a two-stage process. The first stage involves non-enzymatic random hydrolytic chain scission of ester bonds3. The second stage is characterized by the onset of weight loss because of diffusion of oligomers from the polymer bulk. Most of the studies that focus on the degradation after few weeks of immersion (i.e., in vitro) and, in many of them, accelerated degradation through different agents or conditions (such as enzymes, free radicals, pH changes, temperature differences, and alkali) is considered. Compared with PLA and PGA based polyesters, PCL displays low mechanical strength, owing to its good elongation, in combination with its solubility, permeability and feasible processing temperatures (Tg = -60。C). Contrary to PLA, PGA, and polylactic-co-glycolic acid (PLGA), PCL does not undergo plastic deformation and failure when exposed to long cyclic strain thereby making it a suitable component in vascular graft applications4. PCL grafts exhibited good structural integrity in the rat abdominal aorta models5. Considering the aforementioned applications of polyesters, in this study, we provide a new direction for understanding the thin polyester film degradation in the gastrointestinal tract (stomach and intestine) in vivo.
RECENT APPROACH FOR IN VIVO AND IN VITRO DEGRADATION
In vitro degradation studies
A large number of in vitro models have been used to study the polyester degradation and physiology to understand the degradation mechanism. In some of these studies, the objective was to investigate whether long term degradation of polyesters can be applied for the survival of drugs through the6,7 gastrointestinal (GI) tract or to serve as artificial replacements for damaged blood vessels8. Hydrolytic degradation is of crucial importance for its successful implementation in various applications such as surgical sutures, drug delivery systems and tissue engineering scaffolds, and thin films9,10. Hydrolytic rates of the polymer film are found to be similar in both in vitro (saline) and in vivo (rabbit) conditions where the enzymatic involvement is at the first stage of degradation, which is not a significant factor in the degradation process11,12.
The study of hydrolytic degradation of polyesters, whether acid or base catalyzed, is important to understand the pH dependent degradation of biomaterials. The pH of gastric juice in the stomach can go as low as 0.9-1.5, whereas the pH of the pancreatic juice in the duodenum ranges from 7.5-8.213. These polyester films degraded at strong basic conditions (pH= 13) with large cavities observed because of non-uniform superficial erosion; however, at higher acidic conditions (pH = 1), cavities, cracks and fissures appeared, which were consistent with bulk degradation14. In addition, long term degradation of PCL scaffolds using phosphate-buffered saline (PBS) at pH 7.4 also followed a bulk degradation pattern15. PCL polyesters are easily affected by the enzymatic degradation in the GI tract, where the simulated intestinal fluids containing enzymes like lipase significantly enhance hydrolytic bulk degradation in PCL, which is negligible in the other polyesters.
Enzymatic degradation of polyesters
Lipase is an extracellular hydrolytic enzyme that digests aliphatic polyesters such as PCL16,17. Extensive studies targeted at enzymatic degradation, where the degradation of PCL is enhanced by the action of lipase PS in which the crystallinity of PCL film decreased continuously, and the degradation took effect not only in the amorphous phase, but also in the crystalline region18,19. The effects of molecular weight, crystallinity, and morphology on the microbial and enzymatic degradation of thin polyester films have been reported in previous studies; it is observed that the degradation begins with the amorphous regions prior to the crystalline regions20,21,22.
In vivo degradation studies
It is noteworthy that among the more than thousands of papers published during the last decade for biomaterials and tissue engineering, in which polyesters are used in the form of films and scaffolds, only a few have discussed their degradation kinetics, despite the fact that these degradation behaviors are quite different in the in vivo conditions. Electrospun PCL non-woven material was investigated both in in vitro (performed in Ringer solution)23 and in vivo studies (subcutaneous implantation in rats); the results in a study by Li et al. clearly prove that faster degradation occurs in vivo due to enzymatic degradation along with hydrolytic degradation24. In rabbits, the mechanism of biodegradation of PCL, PLA, and their copolymers are qualitatively similar25. However, the rate of the first stage of the degradation process, i.e., non-enzymatic random hydrolytic chain scission, varied by an order of magnitude and was dependent on morphological as well as chemical effects.
Whilst results about degradation in polyesters are undoubtedly of some interest, the in vivo correlations in the GI tract are extremely important. The intestinal environment presents significant challenges to the selection of appropriate materials for various clinical applications including tissue engineering. Polyesters show good biocompatibility and degradation in major applications including stents, sutures, drug delivery, and fixation devices26. Biodegradable polyester stents have a significant advantage, as they do not require any removal process and are dissolved by the hydrolytic enzyme avoiding the problems associated with current therapies for GI obstruction27. Polyester scaffolds regenerate the cellular alignment of native intestinal circular and longitudinal smooth muscle layers28. The features of these polyester materials are also crucial for the production of tissue engineered intestines, which include designed levels of biodegradability, strength, and elasticity comparable to that of the small intestine.
Advantages and Limitations of this protocol
This work presents the significant study of degradation of polymer film in vivo. Implantation enables the inoculation of the polymer film into the stomach and intestine through an external route, which is clinically more relevant compared with the other models that use in vitro. In addition, physiologies can be studied more accurately in the case of in vivo degradation when compared with the other models which are done in vitro. This is of utmost importance when monitoring the degradation of polyester film in GI tract for a short term. Furthermore, this procedure allows systemic and selective suturing ways for clinical work. Moreover, the model allows analysis of clinically important phenomena such as localized host function in a particular area where the degradation takes place. This model of work in in vivo is the first to describe the polymer biofilm degradation in the GI tract.
However, this model also presents some limitations. Besides being an expensive technique, it requires extensive work to acquire more technical expertise. This method requires regular monitoring and manipulations to notably avoid the sacrifice of many hosts.