Indentation: A simple, nondestructive method for characterizing the mechanical and transport properties of pH-sensitive
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We use instrumented indentation to characterize the mechanical and transport behavior of a pH-sensitive hydrogel in various aqueous buffer solutions. In the measurement, an indenter is pressed to a fixed depth into a hydrogel disk and the load on the indenter is recorded as a function of time. By analyzing the load–relaxation curve using the theory of poroelasticity, the elastic constants of the hydrogel and the diffusivity of water in the gel can be evaluated. We investigate how the pH and ionic strength of the buffer solution, the hydrogel cross-link density, and the density of functional groups on the polymer backbone affect the properties of the hydrogel. This work demonstrates the utility of indentation techniques in the characterization of pH-sensitive hydrogels.
I. INTRODUCTION
Hydrogels are cross-linked polymer networks swollen in an aqueous solution. A number of hydrogels are stimuliresponsive; they swell in response to external stimuli, such as temperature,1 electrical field,2 light,3 and pH.4 Such responsive materials are useful in the development of microfluidic devices,5,6 actuators and sensors,7,8 media for cell immobilization,9 and carriers for drug delivery.10–13 Optimized design of these materials requires a quantitative understanding of how the mechanical and transport properties of the stimuli-responsive hydrogels change as the environment changes. A powerful and convenient characterization technique is necessary. Among the various stimuli-responsive hydrogels, the most widely used are pH-sensitive gels because of their practical applications.5–9,13 Hydrogels that are pH-sensitive contain acidic (e.g., acrylic acid) or basic (e.g., tertiary amine) functional groups that donate or accept protons in response to changes in the pH of the environment. For instance, hydrogels containing tertiary amine groups (as does the material used in this research) become protonated when the pH of the surrounding medium falls below the pKa value of the conjugate acid of the amine groups. To maintain charge neutrality, counter ions migrate into the hydrogel together with the protons. The increased concentration of counter ions in the gel provides an osmotic driving force for the hydrogel to swell. The swelling ability of the hydrogel depends on the crosslink density of the gel, the salt concentration of the external solution, and the density of the tertiary amine groups. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.368 152
J. Mater. Res., Vol. 27, No. 1, Jan 14, 2012
The mechanical and transport properties of hydrogels have been studied by many researchers using various test methods. Widely used methods for measuring the stiffness of hydrogels include tension,14–16 compression,17–19 bending,20–22 shearing,23 cavitation,24 and rheological methods,25–27 as well as nano-, and macroindentation.28–38 The accessible length scale and stiffness range for each method are summarized in Table I. Poisson’s ratios of hydrogels are rarely measured. When a load is suddenly
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