Rheologically measured viscosity parameters as indicators for 3D printability of hydrogels

Abstract

3D printing is an additive manufacturing technique, where a three dimensional structure is manufactured layer by layer. Hydrogels are three dimensional networks formed from crosslinked polymers that are capable of absorbing large amounts of water. For medical applications, 3D printing enables the versatile production of hydrogels with customized shapes and high printing quality. The printed structures can be cell-free or contain cells.

In this thesis is presented suitable 3D printing methods for hydrogels. There is also propound rheological test methods, which can provide information about the behavior of the materials when they are stressed. The utilized printing method sets limitations on the printing ink that can be used. One of the most significant properties is the rheological behavior, which can be studied with rheological test methods, such as rotational and capillary rheometers. The capillary rheometer is suitable for studying the behavior of materials at high shear rates and the oscillatory rheometer is suitable for studying viscoelastic properties of the material. In addition, in this thesis is also discussed how rheology measurements can be utilized in 3D printing of hydrogels.

The behavior of inks and the properties of the hydrogel structure are studied so that the printing process is successful and the manufactured product is as suitable as possible for its intended use. Rheological measurements reveal important properties of the material that can be utilized in the printing process. Mathematical models can be used to model the behavior of materials under load and can also be used to determine viscosity parameters that can be utilized in 3D printing. The results gives information about the properties of the inks and tells are they suitable for printing.

Viscosity is an important factor in the suitability of the ink for the printing process. Higher viscosities can be used in extrusion printing, while in droplet, stereolithography and laser-induced forward transfer printing, the viscosity should be relatively low. In the two-photon absorption printing there are no limitations on viscosity. However, a suitable viscosity is not the only important property to determine the suitability of the ink. The change in viscosity, gelation and mechanical properties of the material have an effect during and after the printing process, in order to print structures with the most accurate shape possible.