Hydrogel Stiffness

Stiffness and Tissue Function

The mechanical properties of the extracellular matrix are important for the regulation of cell behavior (1) and other tissue functions. Therefore, matrix stiffness is an important experimental parameter for in vitro 3D cell cultures.
3-D Life Hydrogels can be formed with stiffnesses of up to a 10 kPa dynamic shear modulus. Therefore, the stiffness of a broad range of mammalian tissues (2)⁠ can be modeled.
Both, the thio-reactive polymers and the thiol-containing crosslinkers of the 3-D Life System, which are the basic reagents for forming hydrogels, are supplied as highly concentrated stock solutions. The stiffness of the hydrogels can easily be varied by adjusting the final concentration of these two reagents.

Stiffness of 3-D Life Slow-Gelling Hydrogels

Slow gelling hydrogels (SG series) are used for a broad range of applications, with a soft to medium high stiffness, because the gelation time is long enough to easily handle the solutions. Figure 1 shows gel stiffnesses and corresponding concentrations of reactive groups of the polymers.
Gel formation starts at a concentration of 1 mmol/L reactive groups with a very low stiffness of 20-100 Pascal (Pa). With increasing concentrations of reactive groups, the stiffness of the hydrogels increases up to 10 kPa at a concentration of 10 mmol/L reactive groups. Below 4 mmol/L, the PVA-based hydrogels were softer than the dextran-based gels. The shear loss modulus (G''), a measure for the viscosity of the hydrogel, remained very low (below 2% of the G') in all combinations (data not shown).

slow gelling rheology
Figure  1: Hydrogel stiffnesses and corresponding concen­trations of reactive groups of gel components for slow gelling hydrogels (SG series). The slow-gelling polymers SG-Dextran or SG-PVA were combined with the crosslinkers PEG-Link or CD-Link at pH 7.2 at various concentrations of each reactive group. For example, at 2 mmol/L, the final concentration of thiol-reactive groups of the polymer was 2 mmol/L and the concentration of thiol groups of the crosslinker was also 2 mmol/L. The shear storage modulus (G') of each hydrogel was measured 6 hr after the mixing of the hydrogel components, when the crosslinking reaction was essentially complete.

Stiffness of 3-D Life Fast-Gelling Hydrogels

fast gelling rheology
Figure 2:  Hydrogel stiffnesses and corresponding concen­trations of reactive groups of gel components for fast gelling hydrogels (FG series). The fast-gelling polymers FG-Dextran or FG-PVA were combined with the crosslinkers PEG-Link or CD-Link at pH 5.5 at various concentrations of each reactive group. For example, at 2 mmol/L, the final concentration of thiol-reactive groups of the polymer was 2 mmol/L and the concentration of thiol groups of the crosslinker was also 2 mmol/L. The shear storage modulus (G') of each hydrogel was measured 20 min after the mixing of the hydrogel components, when the crosslinking reaction was essentially complete.

3-D Life Fast Gelling Hydrogels (FG series) are mainly used for the generation of soft hydrogels, since at higher stiffnesses, the gelation time becomes too short for a practical handling. Exceptions are automated pipetting and bioprinting applications. Figure 2 shows the stiffnesses of hydrogels formed at increasing concentrations of fast reacting polymers (FG series) and crosslinkers.
At a concentration of 1.5 mmol/L of each reactive group, all polymer combinations develop a very low hydrogel stiffness (an elastic shear modulus (G') below 100 Pa). This is markedly softer than the stiffnesses obtained from slow gelling gels at the same concentrations (compare Fig. 1). Below 6 mmol/L, the fast-gelling PVA-based hydrogels were softer than the fast-gelling dextran-based gels. At higher concentrations the stiffness increases up to 4-5 kPa. The shear loss modulus (G''), a measure for the viscosity of the hydrogel, remained very low (below 2% of the G') in all combinations (data not shown).

Stiffness of 3-D Life Hyaluronic Acid-based Hydrogels

3-D Life Hyaluronic Acid-based Hydrogels (HA series) provide a natural extracellular matrix component, Hyaluronan, as a major component of 3-D Life Hydrogels.
HA hydrogels can be used in a broad stiffness range from soft to medium stiff and the rate of gel formation is slighly faster than the rate of gel formation of slow-gelling hydrogels (SG series). Figure 3 shows the stiffnesses obtained when of SG-Dextran was crosslinked with the HA-based crosslinkers HyLink and CD-HyLink and when SG-PVA was crosslinked with HyLink.
Gel formation was detected at crosslinking concentrations of 1 mmol/L and higher. At a concentration of 1 mmol/L of each reactive group, both types of crosslinkers develop a very low hydrogel stiffness (a shear modulus (G*) below 100 Pa). At higher crosslinking concentrations the stiffness increases roughly in a similar pattern by both crosslinkers up to several thousand Pa at 5-6 mmol/L.

Rheology of hyaluronic acid-based hydrogels
Figure 3:  Hydrogel stiffnesses and corresponding concentrations of reactive groups of gel components for hyaluronic acid-based hydrogels (HA series). The thioreactive polymer SG-Dextran was combined with the crosslinkers HyLink or CD-HyLink at various concentrations of each reactive group. For example, at 2 mmol/L, the final concentration of thiol-reactive groups of the polymer was 2 mmol/L and the concentration of thiol groups of the crosslinker was also 2 mmol/L. The shear modulus (G*) of each hydrogel was measured 6 hr after the mixing of the hydrogel components, when the crosslinking reaction was essentially complete. At crosslinking concentrations of 1-3 mmol/L hydrogels were prepared at pH 7.2. For higher crosslinking concentrations a pH of 6.2 was used to slow down the crosslinking reaction to allow loading of the pre-gel into the rheometry setup.

Literature

  1. Wells RG. The role of matrix stiffness in regulating cell behavior. Hepatology. 2008 Apr; 47(4):1394-1400.Wells RG. The role of matrix stiffness in regulating cell behavior. Hepatology. 2008 Apr; 47(4):1394-1400.
  2. Chen WLK, Simmons CA. Lessons from (patho)physiological tissue stiffness and their implications for drug screening, drug delivery and regenerative medicine. Adv Drug Deliv Rev. 2011 Apr 30; 63(4-5):269-276.
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