Water world: medical applications of hydrogels

Jason Smith, senior analytical chemist, Polymer Solutions, looks at understanding necessary analytical techniques of hydrogel

A hydrogel is like a molecular sponge.  In looking at a hydrogel material one sees what looks like a solid form. However, the hydrogel has tiny pores throughout its structure that are interconnected to create a vast network of pathways for water and other small molecules to be absorbed and retained. Common uses of hydrogel materials include wound care management, aiding in tissue repair, drug delivery, cartilage replacement, and as the material of choice for contact lenses. While hydrogels have been utilised within the life sciences industry for over 50 years their increased use is poised to grow as these materials are better understood and tailored to have novel functional characteristics. Chemical analysis and physical testing of hydrogels ensures that the material will perform as expected and most importantly that it will be safe for the patient.

According to Gulrez et al (2011) there are three important characteristics for a hydrogel. These characteristics are the hydrogel’s water holding capacity and permeability, dissolution rate, and its biocompatibility. 

First, the hydrogel must have the ability to hold water to be effective.  Therefore knowing its capacity lets you know how much water the hydrogel bonds to the hydrophilic groups and how much water is bound through the materials hydrophobicity. 

Secondly, when the hydrogel is in place it needs to either deliver or direct changes in the system. This means that small molecules need to migrate out of the hydrogel, as in drug delivery of active pharmaceutical ingredients (API), or that the hydrogel needs to dissolve, as in keeping an area moist for tissue repair. The dissolution rate itself must also be reliable and appropriate for the specific application.

Thirdly, the biocompatibility needs to be understood to ensure that the hydrogel can be degraded within the body without producing any toxic substances.  The dissolution products should be processed normally by the body without complications. 

Properties of hydrogels

Defining the hydrogel properties must be done carefully and methodically during the R&D phase, with analytical testing to confirm material performance. Once the desired properties are defined for a hydrogel, it is important to evaluate the quality of the end product through proper testing protocols.  Validated test methods should be followed to assure the hydrogel will meet all critical performance specifications. There are multiple analytical options that should be considered for the testing of hydrogels during development, implementation, and commercial production.

Fourier Transform Infrared (FTIR) Spectroscopy is a non-destructive technique that allows the user to identify molecular species in a sample matrix.  In order to determine if the correct polymer system is being used in the hydrogen, FTIR spectroscopy is employed to confirm the proper molecular structure.  FTIR spectroscopy can also be used to confirm the identity of any other species that are present in the hydrogel material whether or not they are meant to be in the matrix.

Gel Permeation Chromatography (GPC) is utilised to determine the molecular weight distribution of a polymer or macromolecule. For hydrogels GPC is used to determine the molecular weight distribution of the non-crosslinked polymer fraction of the hydrogel.  This analysis gives an indication of the crosslinking efficiency for the hydrogel polymer fraction.

Microscopy techniques can be used to look at surface characteristics of hydrogels. More specifically, Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) can be employed to look at surface of the hydrogel and to confirm composition of additives to the hydrogel matrix. Whereas TEM is utilised to look at structure, pore size, and dispersion of fillers in a hydrogel.  This analysis can be utilised with a quality assurance application to ensure that the end product is uniform.

Water holding capacity

Hydrogel water holding capacity is an important property to understand fully, this is quantifying the components of the hydrogel and determining the amount that is water and the amount that is gel.  This technique is done with a wet chemistry technique of immersing the hydrogel in water or mildly alkaline water and then drying off the water.  The fraction that remains is the hydrogel content of the material.

Swelling determines how much water a hydrogel can hold. This is particular useful for wound care applications in which the hydrogel needs to absorb large amounts of fluid. It is also very relevant to industries that use hydrogels for hygiene applications such as adult incontinence products. To determine this, a hydrogel dried to it polymer fraction and then allowed to absorb water for twenty four hours and the weight change is calculated as the amount of swelling.

Importance of patient safety

Patient safety is always a critical consideration when developing products for the life sciences industry. After a hydrogel is produced it must be tested to ensure that no residual or unwanted compounds remain in the matrix that will be exposed to body tissues.  There are impurities that could have a toxic effect at very low concentrations. Extracting the hydrogel in water or other suitable solvent and subsequent analysis is prudent.  Extracts can be analysed by several chromatographic techniques to determine the molecular structure and concentration of residual solvents and chemicals that are present in the hydrogel matrix. Chromatographic techniques that can be utilised include high performance liquid chromatography (HPLC), liquid chromatography mass spectroscopy (LCMS), gas chromatography (GC), and gas chromatography mass spectroscopy (GCMS).   

If the hydrogel is designed for drug delivery it must be tested for the presence and amount of the medication.  The drug must be extracted with a suitable solvent and then identified and quantified by the appropriate technique such, as FTIR or LCMS or GCMS.  Another important parameter to measure is the rate of drug extraction. If the drug is to be time released, leachables testing is utilised to determine the rate at which the medicaments or active pharmaceutical ingredients migrate out of the hydrogel matrix.

As the use of varying types of hydrogels increase, there is a need to define testing procedures that ensure the safe use of these useful polymer matrices. Understanding the structure and functionality that they contain is important to build the proper knowledge base. Hydrogels are poised to improve quality of life and promote better patient outcomes but can only do this when supported by great analytical science and the proper laboratory techniques.

REFERENCES

Syed K. H. Gulrez, Saphwan Al-Assaf and Glyn O Phillips (2011). Hydrogels: Methods of Preparation, Characterisation and Applications, Progress in Molecular and Environmental Bioengineering - From Analysis and Modeling to Technology Applications, Prof. Angelo Carpi (Ed.), ISBN: 978-953-307-268-5, InTech, Available from: http://www.intechopen.com/books/progress-in-molecular-and-environmental-bioengineeringfrom-analysis-and-modeling-to-technology-applications/hydrogels-methods-of-preparation-characterisationand-applications