Testing the viscoelasticity of 3D printed hydrogels using ElastoSens™ Bio
— The mechanical characterization of 3D printed scaffolds have been conventionally performed by destructive testing techniques.
— ElastoSens™ Bio has shown to provide reproducible and sensitive measurements of the viscoelastic properties of 3D printed scaffolds.
— The volume fraction and the printing pattern contributed to the mechanical properties of 3D constructs.
3D printing technologies offer the advantage of precisely controlling the microstructure of scaffolds used for tissue engineering applications and drug delivery systems. The macro-mechanical properties of these scaffolds are directly related to their microstructure and both are important parameters for cell behavior and drug release [1,2]. The evaluation of the scaffold mechanical properties has been conventionally performed by destructive, in contact testing techniques (e.g. compression and tensile tests). This prevents the use of the 3D printed scaffold for further characterizations and requires multiple samples to test its mechanical stability over long periods of time. In addition, technical limitation of conventional instruments are usually an issue to precisely measure the soft nature of these scaffolds. In this short application note, the viscoelastic properties of two 3D printed scaffolds made of two different materials were tested using the ElastoSens™ Bio.
MATERIALS AND METHODS
RTV silicone rubber (Dow Corning, MI, USA) and poloxamer gel (Allevi, PA, USA) were 3D printed inside the ElastoSens™ Bio sample holder (Fig. 1) to produce scaffolds with different porosities (volume fraction). The Bioscaffolder printer (Analytik, Cambridge, UK) and the Allevi 2 printer (Allevi, PA, USA) were used for printing the silicone and the poloxamer gels, respectively. Scaffold’s volume fraction was calculated as follows:
The printing patterns of the 3D printed silicone scaffolds are shown in Fig. 2. The structure varied in volume fraction (100 %, 75 %, 68 % and 54 %) by adjusting the line width (0.5 mm and 0.8 mm) and the void width (1.16 mm, 0.69 mm and 0.89 mm). A thin layer of silicone was printed between the sample and the holder to ensure their contact in order to meet the testing requirements of the ElastoSens™ Bio.
After the complete gel formation, water was added into the sample holders containing the scaffolds in order to replace the air in the pores. A vacuum chamber was used to produce a negative pressure and force air to be expelled from the scaffold pores. By doing so, the macro density of the scaffold was close to 1.0 (a requirement of the testing method). The scaffolds were then tested in the ElastoSens™ Bio.
RESULTS AND DISCUSSION
For the poloxamer scaffolds, the storage modulus also decreased when porosity increased as a direct consequence of the lower amount of polymer present in the construct (Fig. 5) (lower volume fraction). Printed scaffolds with 47 % and 43 % of volume fraction had a decrease of 85 % and 88 % in the storage modulus when compared to the bulk gel, respectively.
— The direct printing inside the sample holder of ElastoSens™ Bio avoids the excessive manipulation of soft and fragile constructs which can cause sample damage and contamination.
— ElastoSens™ Bio is an easy-to-use, non-destructive and contact free instrument that measures the viscoelasticity of 3D printed scaffolds.
—The system offers the possibility to easily tune the mechanical properties of 3D printed scaffolds by altering the composition of the bioink as well as the printing pattern .
— Testing the mechanical evolution of the same sample over short or long periods of time is now possible thanks to the non destructive nature of ElastoSens™ Bio.
— 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.
 Woo Lee, J., & Cho, D. W. (2015). 3D Printing technology over a drug delivery for tissue engineering. Current Pharmaceutical Design, 21(12), 1606-1617.
 Chen, Z., Zhao, D., Liu, B., Nian, G., Li, X., Yin, J., … & Yang, W. (2019). 3D Printing of Multifunctional Hydrogels. Advanced Functional Materials, 29(20), 1900971.
 Godau, B. (2019). Determining the effect of structure and function on 3D bioprinted hydrogel scaffolds for applications in tissue engineering (Master dissertation).