Thermal degradation analysis of 3D printed scaffolds using ElastoSens™ Bio

SUMMARY

The evaluation of the thermoreversible behavior of specific hydrogels is conventionally performed with in-contact and destructive techniques.
ElastoSens™ Bio characterizes thermoreversible properties through the measurement of the viscoelastic properties of thermoreversible hydrogels.
The thermoreversible properties of gelatin and poloxamer bulk gels and 3D printed scaffolds were observed by the pronounced change in the shear storage modulus as a function of temperature.
The structure and the concentration of the thermoreversible gels influenced the gel formation and degradation kinetics.

INTRODUCTION

The thermoreversible behavior of some polymers relies on the large conformation changes in response to temperature. They have been investigated for a variety of clinical applications that demand an in situ gelation at physiological temperatures. In addition, these polymers have been widely studied for other biomedical applications such as drug delivery and tissue engineering in which the thermoresponsive behavior needs to be balanced with biocompatibility and degradation kinetics [1]. In this field, thermoreversible polymers have also been used to produce sacrificial bioinks for 3D printing. The idea of these bioinks is that they can be washed away after printing. Therefore, they can offer temporary support or can be used to create complex geometries within a structure for vasculature networks [2]. The analysis of the thermoreversible behavior of these gels is usually performed by evaluating the evolution of its mechanical properties. In most cases, destructive tests are conventionally employed requiring the use of multiple samples to get measurements over long periods of time. In addition, technical limitations of conventional instruments are usually an issue to detect subtle changes in the soft viscoelastic nature of these hydrogels. In this short application note, ElastoSens™ Bio was used to measure the viscoelastic properties of bulk thermoreversible polymers (gelatin and poloxamer) at different temperatures as well as the degradation of 3D printed scaffolds composed of the same thermoreversible hydrogels.

MATERIALS AND METHODS

Fig 1: 3D printing of gelatin in the ElastoSens™ Bio sample holder

Bovine gelatin (Sigma-Aldrich, MO, USA) dissolved in deionized water (20 %, w/w) and Poloxamer (BASF, Germany) prepared at different concentrations (20.0 % and 17.5 %, w/w) were 3D printed directly in the ElastoSens™ Bio sample holder using an Allevi 2 bioprinter (Fig. 1 and Fig. 2, respectively). A thin layer of gel was printed between the sample and the holder to ensure their contact in order to meet the testing requirements of the instrument. Gelatin scaffolds were tested at 45 °C for 7 minutes and compared under the same conditions. For the poloxamer scaffolds, two tests consisted of: (1) increasing the temperature from 5 °C to 50 °C for bulk samples, and (2) applying a constant temperature (12 °C) for 10 minutes for the 3D printed scaffold, were performed.

Fig. 2: Schema of the 3D printed poloxamer scaffold in the ElastoSens™ Bio sample holder

RESULTS AND DISCUSSION

Fig. 3 shows the evolution in the shear storage modulus (G’) of the gelatin gels during 7 minutes at 45 °C. For both conditions (3D printed scaffold and bulk gel), the shear modulus decreased over time confirming the complete thermal degradation of the gelatin. The 3D printed construct showed a considerably lower initial G’ (15 % of the G’ from the bulk gel) which was expected since the first is a porous structure. Another noticeable difference between the two samples was the degradation rate which was substantially higher for the bulk gel implied by the steeper slope of the curve. This difference shows that the structure influences the degradation process.

Fig. 3: Elastic Shear Modulus evolution during time at 45 °C.

Fig. 4 shows the influence of temperature on the shear storage modulus for the 20.0 % and 17.5 % (w/w) poloxamer scaffolds. The results confirmed the thermoreversible nature of poloxamer through the pronounced increase in the storage modulus between 12 °Cand 25 °C (sol-gel transition). The more concentrated condition started the transition at lower temperatures because of the higher amount of polymer chains available for the self-aggregation into micelles which in turn aggregate into the hydrogel [3]. For temperatures higher than 15 °C, the condition of 20 % (w/w) showed higher storage modulus than the condition of 17.5 % also due to the higher amount of polymer present in the gel. For temperatures higher than the transition range, the results showed the thermal degradation of the polymer. The increase in temperature accelerated its degradation.

Fig. 5 shows the inverse transition from bulk gel to solution of the 3D printed poloxamer at 12 °C. This was confirmed by the substantial decrease in the Shear storage modulus (G’) during the 10 minutes of the test.

Fig. 4: Shear storage modulus (G’) as a function of temperature for poloxamer scaffolds at 20 % (w/w, orange line) and 17.5 % (w/w, blue line).

Fig. 5: Shear Elastic Modulus as a function of time of 3D printed poloxamer gels at 12 °C.

CONCLUSION

The 3D printed gelatin scaffold showed a considerably lower initial G’ when compared to the bulk gel. For both conditions, the shear modulus decreased during the 7 minutes of the study at 45 °C confirming the complete thermal degradation of the gelatin. The bulk gel had a higher rate of degradation, showing that the structure influences this process. The variation of temperature during the viscoelasticity testing of the poloxamer scaffolds showed their thermoreversible properties by the pronounced change in the shear elastic modulus between 12 °C – 25 °C. The higher concentration of polymer in the gel was responsible for a transition at lower temperatures. The inverse transition of 3D printed poloxamer gels into solution at 12 °C was observed by the substantial decrease in the shear storage modulus during the 10 minutes of the test.

PERSPECTIVES

— ElastoSens™ Bio allows to rapidly vary the temperature in the same test for studying the thermoreversible behavior in real time of specific polymers through their viscoelasticity.

— ElastoSens™ Bio is able to capture subtle mechanical changes during gel formation and degradation.

— The direct printing inside the sample holder of the ElastoSens™ Bio avoids the excessive manipulation of soft hydrogels which can cause sample damage and contamination.

— ElastoSens™ Bio allows testing the viscoelasticity of biomaterials under different physical (e.g. photo or thermo stimulation), chemical (e.g. crosslinking solution) and physiological (e.g. enzymatic solution) conditions to simulate in vivo behaviors.

REFERENCES

[1] Hogan, K. J., & Mikos, A. G. (2020). Biodegradable thermoresponsive polymers: Applications in drug delivery and tissue engineering. Polymer, 211, 123063.

[2] Prendergast, M. E., Solorzano, R. D., & Cabrera, D. (2017). Bioinks for biofabrication: current state and future perspectives. Journal of 3D printing in medicine, 1(1), 49-62.

[3] Furth, M. E., Atala, A., & Van Dyke, M. E. (2007). Smart biomaterials design for tissue engineering and regenerative medicine. Biomaterials, 28(34), 5068-5073.

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