Freeze-drying is a common method to produce porous scaffolds from a hydrogel composed of natural or synthetic polymers. In a number of cases, the polymeric solution is crosslinked (bonds or interactions are established among the polymeric chains) and the hydrogel is then freeze-dried to obtain the porous and dried scaffold (1-3). This crosslinking phase can take a few minutes to several hours to complete, and the time point in which the crosslinking conditions are forcibly ceased and the sample transferred to the freeze-dryer has a significant impact on the final structure of the dried scaffold. This transition from the crosslinking phase to the freeze-drying is often determined visually, and the lack of precision in this point leads to a lack of consistency in the final dried scaffolds which can considerably decrease the production yield. In this technical note, the use of the ElastoSens™ Bio to guide the determination of this optimal transition point will be described.
The crosslinking phase from a polymeric solution to a formed hydrogel is a process dependent on time in which the number of interactions among the chains continuously increases (as long as the required conditions of pH, light, temperature, etc. are maintained) until a maximum is reached. This is directly related to the stiffness of the forming hydrogel: the increase in the number of interactions leads to a continuous increase of the hydrogel stiffness up to a maximum stable level. The ElastoSens™ Bio is able to measure in real time the stiffness (shear elastic modulus, G’) of a sample as a function of time and therefore obtain its complete crosslinking curve (or kinetics) (Figure 1).
Figure 1: Crosslinking kinetics of a hydrogel in terms of shear storage modulus (G’). The number of interactions among the polymeric chains increases with time and so the (G’). An optimal dried scaffold is dependent on the stiffness of the hydrogel when the freeze-drying process is initiated.
Figure 2: Crosslinking kinetics of a hydrogel in terms of shear storage modulus (G’). Two batches of the same hydrogel can reach a certain level of stiffness at different times.
After some iterations, the user can determine the optimal stiffness level (optimal point or optimal window) in the crosslinking curve obtained by ElastoSens™ Bio that will lead to the optimal scaffold architecture (porosity, pore size, interconnectivity of pores, homogeneity, mechanical properties) for the given process (Figure 1). After this determination, an aliquot of the polymeric solution can be tested in the instrument at each production cycle to ensure that similar levels of stiffness were reached before passing to the freeze-drying step. This precise transition from the crosslinking phase to the freeze-drying step has been showing to be extremely important to obtain high production yields in the industry. In addition, this can accelerate research and development of freeze-dried products.
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