Rheology of Hydrogels

Sarah Cotts | Morgan Ulrich
June 20, 2022

Characterizing and Applying Hydrogels

Hydrogels are three-dimensional porous strctures that can absorb large amounts of water. They can be made up of polymers, protein, peptides, colloids, surfactants, or lipids.1 Hydrogels’ ability to uptake large amounts of water is useful for many biological applications, including drug delivery and tissue engineering. Since a hydrogels’ properties change as it absorbs water, scientists must accurately characterize its behavior at different saturation amounts and in varying conditions.  

An example of a naturally occurring hydrogel is collagen, the main structural protein in the human body. Collegen is an ideal material for tissue structures due to its fibrous nature – its molecules are tightly packed together in long fibrils and, depending on the exact structural arrangement, can produce fibrils with similar tensile strengths to steel but with some flexibility. 

Collagen’s remarkable structural properties and biocompatibility lead to it becoming a widely used material, particularly as a scaffold in tissue engineering.2 Since collagen is a natural material, its qualities vary from sample to sample. Therefore it can be challenging to manufacture collagen-based products with reproducible properties. However, the inherent variability of natural starting materials is not an insurmountable challenge – it simply requires the correct material property testing to be performed to predict the material’s behavior for the final application.  

Throughout this blog we will use collagen as our primary example of a hydrogel to characterize. However, these same techniques can be applied to any hydrogel of any source material. 

Rheology of Hydrogels

Rheology – the measure of flow and deformation behavior of liquids and solids – is an ideal methodology for studying hydrogels such as collagen.  Hydrogels are inherently viscoelastic, meaning they exhibit both viscous and elastic behavior. Dynamic (oscillatory) rheology characterizes these complex materials and reports quantitative measurements of viscoelasticity:  

  • Storage modulus: how much energy must be applied to distort the sample 
  • Loss modulus: the energy lost as the material returns to its original shape after deformation 
  • Tan delta: the ratio of the loss modulus to the storage modulus and the measure of the damping abilities of the hydrogel 

These viscoelasticity measurements are critical when trying to understand the differences in the degree of cross-linking, glass transition, heterogeneity, and molecular weight between hydrogel samples and how it impacts the end-use performance.1 Rheometers easily measure viscoelasticity and are the ideal tool for optimizing these qualities. 

Another important measurement that is critical for hydrogels is yield stress. A lot of hydrogels are injectable materials so yield stress is very important as it determines how the hydrogel is retained at the injection point. The shear response also determines how easy the hydrogel solution is to inject. 

Since different collagen starting materials can have very different physical and thermal properties, researchers must carefully characterize collagen under realistic application conditions to select the best materials for specific uses.3 Rheology test equipment’s rapid screening capabilities makes industrial collagen processing both efficient and cost-effective.  

Choosing the Best Rheometer for Hydrogels

Performing high-quality, reproducible rheology measurements requires the right test instrument. TA Instruments is a world leader in the design and manufacture of test systems for materials properties, including for rheology.  

Our signature rheometer, the Discovery Hybrid Rheometer, is an industry-leading instrument for rheological measurements. The easy-to-use DHR performs a wide range of standard and advanced analysis measurements that are crucial for hydrogel characterization. 

Material Analysis of Bioplastics

In any plastic feedstock, material analysis is crucial for optimizing processing conditions and end-use performance. New bioplastic formulations require even more rigorous testing to ensure that they meet user standards while maximizing sustainability. The following material analysis techniques are already common in plastic development but are even more crucial for successful bioplastic innovation.

Thermal analysis measures how changes in temperature affect a material’s properties. Thermal analysis instruments typically measure heat flow, weight loss, dimension change, or mechanical properties as a function of temperature. Characterizing thermal properties is crucial for bioplastic material selection and optimization, answering questions such as:

  • How will this material behave when heated up for processing, including molding or extrusion?
  • Will the plastic maintain its stability if it’s heated or cooled during transport or use?
  • How will the material decompose? Can we optimize the materials for biodegradable plastics?

The most common thermal analysis instrument for bioplastics is a Differential Scanning Calorimeter (DSC) which measures thermal stability and phase transitions to analyze a plastic’s behavior under different conditions. Bioplastic developers use Thermogravimetric Analysis (TGA) for exact composition determination, including volatile or solvent content and filler content (residue). TGA also measures decomposition temperature as well as resulting products, which are crucial for designing sustainable biodegradable plastics.

Mechanical Analysis includes a variety of techniques to characterize a material’s mechanical properties or determine a structure’s response to force. Bioplastic manufacturers use Dynamic Mechanical Analysis (DMA) to measure storage modulus, loss modulus, tan delta, and glass transition (Tg); these measurements help predict a plastic material’s response to forces and deformations at set temperatures. Material strength (measured with Young’s modulus, yield strength, ultimate strength, elongation at break, fatigue and durability) is crucial for predicting the final product’s behavior and can be efficiently measured on mechanical test load frames.

Rheology is the study of the flow and deformation of materials. Rheometers measure bioplastics’ viscosity, which is crucial which is crucial for optimizing processability especially through extrusion and blow molding. Rheology also supports analysis of plastic’s uniformity and durability while developing a new blend.

These material measurements facilitate every step of bioplastic manufacturing, from evaluating the quality of a given feedstock to testing the properties of a final product. As new bioplastic formulations are developed, manufacturers will need systematic approaches to incorporate new blends while maintaining efficiency, meeting customer expectations, and maximizing sustainability.

 

The Best Material Measurements for Bioplastics

Bio-based and biodegradable plastics pose new challenges, yet manufacturers are already rising to the occasion with cutting-edge material analysis. Accurate measurements of composition and material behavior at every stage of manufacture facilitate the design and production of new green plastic products. Leading bioplastic developers around the world are turning to TA Instruments, the world leader in thermal analysis, rheology, microcalorimetry, and mechanical test instruments. TA Instruments set the industry-accepted standard for measuring material properties and are therefore the brand of choice for top polymer labs.

While adopting a new material test regimen can seem daunting, TA Instruments are designed with the user in mind. Many of our instruments are equipped with TRIOS one-touch functionality to run pre-programmed tests with just the click of a button. We also offer industry-leading customer support to help our users from instrument selection and installation through to successful testing.

Whether you are looking to expand your existing testing capabilities or pivot to meet new sustainability challenges, TA Instruments offers the instrumentation and guidance you need. Contact our experts today for personalized support and answers to any questions. And visit our polymers page to learn more about our unique offerings for the plastics industry.

The Discovery HR

The Discovery HR can perform several standard operating procedures for the quality control of collagen hydrogels, including the evaluation of viscoelasticity, including evaluating under which stress conditions the deformation of the hydrogel starts to become nonelastic4. For hydrogels this type of evaluation is particularly important, as samples that exhibit such viscoelastic properties at very low strain percentages will not be suitable for the final product. The Discovery HR Rheometer makes it possible to rapidly identify candidates best suited for specific medical applications.    

The Discovery Hybrid Rheometer is designed for ease of use across simple and advanced measurements. The instrument is also compatible with the TRIOS AutoPilot software to create automated routines, including measurement, analysis and report generation. For industrial manufacturing, where time is of the essence, this software can help accelerate decision-making and reduce the training time of new users.  

For rheometry measurements, the Discovery HR has a number of temperature systems and temperature chambers, including a relative humidity system which may be useful for measuring hygroscopic hydrogels under different ambient conditions.  

The Discovery HR can perform rotational rheology and linear dynamic mechanical analysis in a single instrument under temperature-controlled conditions. The DHR’s excellent sensitivity enables measurements on materials with a range of hardnesses, from steel to more flexible collagens.  

To learn more about rheological evaluation of hydrogels read our new rheology of hydrogels application note. Personalized solutions and expert advice are just a call or click away – contact us to learn how TA Instruments rheometers can improve your efficiency and quality control when working with complex biomaterials.  

 

References:

  1. Baby, D. K. (2020). Chapter 9 – Rheology of hydrogels. In Rheology of Polymer Blends and Nanocomposites. Elsevier Inc. https://doi.org/10.1016/B978-0-12-816957-5.00009-4 
  2. Dong, C., & Lv, Y. (2016). Application of Collagen Scaffold in Tissue Engineering: Recent Advances and New Perspectives. Polymers, 8(42), 1–20. https://doi.org/10.3390/polym8020042  
  3. Meyer, M. (2019). Processing of collagen based biomaterials and the resulting materials properties. BioMedical Engineering OnLine, 18(24), 1–74. https://doi.org/10.1186/s12938-019-0647-0 
  4. Perez-Puyana, V. et al. (2020). Fabrication and Characterization of Hydrogels Based on Gelatinised Collagen with Potential Application in Tissue Engineering. Polymers, 12(5):1146. https://doi.org/10.3390/polym12051146