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This is a short report of a study performed by Caroline Ceccaldi, Satu Strandman, Eve Hui, Emmanuel Montagnon, Cédric Schmitt, Anis Hadj Henni and Sophie Lerouge at École de technologie supérieure (ÉTS) (Montreal, Canada) entitled Validation and Application of a Non-Destructive and Contactless Method for Rheological Evaluation of Biomaterials and published in 2016 on the Journal of Biomedical Materials Research Part B (105B:2565–2573)
— The measurement of hydrogels degradation through their viscoelastic properties is usually obtained for one specific time point during the degradation process.
— ElastoSens™ Bio has shown to provide precise measurements of the gelation and degradation kinetics of chitosan hydrogels.
— ElastoSens™ Bio allowed the analysis in real time of the enzymatic (papain) degradation of chitosan-based hydrogels.
Biodegradable hydrogels are promising candidates as drug carriers due to their biocompatibility and tunable degradation. This is particularly valuable for oral delivery systems since the polymer should respond to pH or enzymatic changes in the gastrointestinal environment to achieve a controlled drug release. The in vitro evaluation of oral drug delivery systems are performed by immersing a sample in physiological fluids and specific external conditions (such as in fluids of low pH and/or in an enzymatic solution at 37 °C) and measuring the released amount of the therapeutic agent [1,2]. The degradation of hydrogels can also be observed by their weight loss or by the evolution of their viscoelastic properties. Data obtained by traditional mechanical testing instruments usually describe the material at one specific time point during the degradation process and do not give information about the full degradation kinetics. Furthermore, the destructive nature of the conventional techniques prevent the re-use of samples for further characterizations. In this short application note, a chitosan-based hydrogel was prepared and tested during gelation and degradation using ElastoSens™ Bio. Degradation was induced by the hydrolytic action of the papain enzyme to simulate gastrointestinal conditions.
MATERIALS AND METHODS
An aqueous acid solution of chitosan (Marinard Biotech, QC, Canada) was mixed with a gelling agent to induce gelation of the polymer (final concentration 2 % w/w). After rapid mixing, the solution (2 mL) was poured into the ElastoSens™ Bio sample holder, which was then placed in the thermal chamber of the instrument at 37 °C. A thin film of distilled water was added to cover the sample and avoid drying. The measurement of viscoelastic properties was started 10 min after the addition of the gelling agent and continued for 60 min during the gelation of the chitosan. After this, the digestion solution (acetate buffer at pH 4 containing 5 mM L-cysteine and 0.5 U/mL papain) was added on top of the samples and removed after 10 minutes leaving enough time to diffuse and induce the degradation. Then, the mechanical measurements were continued for another 60 min to obtain the degradation curve. Reference samples were studied using the same protocol but adding distilled water instead of digestion solution .
RESULTS AND DISCUSSION
Fig. 1 shows the evolution of the shear storage modulus (G’) and loss tangent (tan(δ)=G’’/G’) of the chitosan-based hydrogel as a function of time over 60 minutes. The results show an increase in the shear storage modulus of the hydrogel and a decrease in the loss tangent with time accompanying a gradual shift from viscous liquid (tan(δ) ~ 1) to soft viscoelastic solid (tan(δ) ~ 0). Fig. 2 shows the temporal evolution of the shear storage modulus (G’) of chitosan hydrogel samples that have been exposed to the digestion solution (papain) or distilled water. The gelation kinetics during the first 60 min was nearly identical for both samples. Adding distilled water did not change the gelation process and storage modulus of the reference sample continued to increase (increase in G’ by 70 % in 1 h). However, when the papain solution was added, the sample started to lose its mechanical strength (decrease in G’ by 25 % in 1 h). Chitosan, which is a copolymer of glucosamine (GlcN) and N-acetylglucosamine (GlcNAc) linked by beta-1,4-glycosidic bonds, can be degraded by nonspecific enzymes. Among such enzymes, papain depolymerizes chitosan efficiently, mainly on GlcN-GlcNAc bonds, yielding low-molecular-weight chitosans and chitooligosaccharides. As a result, the solubility of chitosan increases and its viscosity decreases. In a hydrogel, this leads to reduced mechanical properties and eventually dissolution of the gel. We can conclude that the decrease in mechanical properties measured by ElastoSens™ Bio was directly related to the degradation initiated by the action of the enzymatic solution. The instrument was able to measure this effect without altering the microstructure of the hydrogel.
Fig. 2: Shear storage modulus (G’) as a function of time for chitosan-based hydrogels (orange line, gelation kinetics) and chitosan-based gels with papain digestion solution (turquoise line, degradation kinetics).
— ElastoSens™ Bio is able to capture subtle mechanical changes in real time during the degradation of hydrogels in contact with digestion solutions
— ElastoSens™ Bio is an easy-to-use instrument that measures the viscoelastic evolution during gel formation and degradation.
— Thanks to its contact-free testing technology, the ElastoSens™ Bio allows the exposition of hydrogels and biomaterials to simulated physiological conditions such as enzymatic solutions.
 Sharpe, L. A., Daily, A. M., Horava, S. D., & Peppas, N. A. (2014). Therapeutic applications of hydrogels in oral drug delivery. Expert opinion on drug delivery, 11(6), 901-915.
 Wooster, T. J., Acquistapace, S., Mettraux, C., Donato, L., & Dekkers, B. L. (2019). Hierarchically structured phase separated biopolymer hydrogels create tailorable delayed burst release during gastrointestinal digestion. Journal of colloid and interface science, 553, 308-319.
 Ceccaldi, C., Strandman, S., Hui, E., Montagnon, E., Schmitt, C., Hadj Henni, A., & Lerouge, S. (2017). Validation and application of a nondestructive and contactless method for rheological evaluation of biomaterials. Journal of Biomedical Materials Research Part B: Applied Biomaterials, 105(8), 2565-2573.
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