Elucidating the effects of hydrogel molding

Rok Simic¹, Yvonne Gombert¹, Thomas Geue², Nicholas D. Spencer¹

¹ETH Zürich, Switzerland, ²Paul Scherrer Institute, Switzerland

INTRODUCTION: Hydrogels, being soft, biphasic and generally very lubricious are widely studied, especially to mimic behavior of various biological tissues [1]. For example, hydrogels are used to fabricate contact lenses as well as to study tribological behavior of cartilage in joints, due to their good lubrication properties [2, 3]. For these applications, however, the ability of hydrogels to confine water within their surface layers plays an important role in determining interactions with their surroundings [1]. The interactions can be tailored by means of surface modification, which can vary from chemical functionalization to using the influence of the mold surface on gel structure and mechanical properties.

Probing hydrogel surfaces, however, is a challenging task due to the sparse and soft polymer network, which traps large amounts of water. This biphasic structure indeed gives hydrogels their unique properties but on the other hand limits the feasibility of available techniques to study these soft and sensitive surfaces.

We have used several surface-sensitive techniques to characterize the surface properties of gels, whose surface layers had been modified by polymerization in two different molds [4]. Macro-tribological sliding tests were performed to elucidate the effect of surface modification on the coefficient of friction as a function of sliding speed. Elastic moduli of the two surfaces in liquid were determined using colloidal-probe nano-indentation, while the polymer densities in the surface region at different contact pressures were assessed using attenuated-total-reflection infra-red (ATR-IR) spectroscopy and neutron reflectometry (NR). The latter enables precise determination of the polymer-density profile in the surface layers with sub-nanometer resolution [5]. It can also provide information on liquid confinement at the surface, which is believed to be one of the main reasons for the ultra-low coefficient of friction during sliding of these materials [6].

METHODS:  The acrylamide (AAm) and the cross-linker N,N’-methylenebisacrylamide (bis-AAm) were dissolved in Milli-Q water at 7.5 wt.% and 0.3 wt.%, respectively. Free-radical polymerization was initiated by ammonium persulfate (APS) and catalyzed by N,N,N’,N’-tetramethylethane-1,2 diamine (TEMED) at a concentration of 0.01 wt.% each. The solution was poured into piranha-cleaned glass petri dishes and into PDMS molds. After polymerization, the gels were immersed in Milli-Q water for at least 48h to remove unreacted reagents and to allow swelling of the gels. The surfaces of the gels that had been in contact with the molds were used for analysis. Friction was measured for self-mated hydrogel pairs during reciprocating sliding of 10 mm hydrogel pins over flat hydrogel surfaces with a stroke length of 5 mm and sliding velocities from 0.1 to 20 mm/s. The contact pressure was set to about 6 kPa, corresponding to the range of pressures between an eyelid and a cornea during a blink of an eye [7]. The elastic moduli in the first couple of mm from the surface were determined by AFM-based nanoindentation. To this end, a silica microsphere (R=13 mm) was attached to the end of a tipless AFM cantilever (k=1.24 N/m) and pressed against the hydrogel surfaces in liquid. The moduli were determined from the force-distance curves using Hertzian contact mechanics. To elucidate the polymer density at the surfaces, the gels were gradually pressed against the ATR-IR crystal in liquid while measuring the intensities of the characteristic peaks. A similar approach was used for the NR study to determine the polymer density near the surface.

RESULTS:  The data in Figure 1a presents the measured coefficient of friction for the self-mated PAAm hydrogels. The friction of the glass-molded hydrogel was always higher than the friction of the PDMS-molded gel and increased substantially with increasing sliding speed. On the other hand, the friction of the PDMS-molded gel remained close to the super-lubricity limit of 0.01 irrespective of the sliding speed. Nano-indentation experiments showed substantial differences in elastic moduli for the both hydrogel surfaces. The Young’s modulus of the glass-molded surface was about 11 kPa and remained relatively constant within 2 mm of the surface, whereas the PDMS-molded hydrogel was much softer and showed more of a gradient in modulus, with an average value of 0.3 kPa over the first few mm of depth. The ATR-IR results showed higher intensity of the amide peaks for the stiffer, glass-molded sample compared to the PDMS-molded one. Increasing the contact pressure from 0.1 to 10 kPa increased the signal intensity for both samples, however, the polymer-density seemed to remain lower for the PDMS-molded gel over the entire range of tested pressures. A similar result was also obtained with the NR, where the polymer density at the surface of the glass-molded sample remained higher than the density for the PDMS-molded sample at both tested contact pressures (0.1 and 10 kPa), Fig 1b.

Figure 1 a) Coefficient of friction as a function of sliding speed for a glass-molded gel and a PDMS-molded gel in self-mated contact at » 6 kPa of contact pressure. b) Scattering-length density profile for the two gels obtained by fitting the neutron reflectivity curves of the two gel surfaces pressed against a silicon block at 0.1 kPa (0.1 N) and 10 kPa (5 N) pressure in water.

DISCUSSION: By polymerizing the PAAm hydrogels against a hydrophilic or a hydrophobic mold, we were able to affect the surface properties of otherwise similar bulk hydrogels. The much lower friction in the case of the gel from the hydrophobic mold implies a softer, brushier surface than in the case of the gel from a hydrophilic mold. This was confirmed by the much lower elastic moduli of the hydrophobically molded gel. Both ATR-IR and NR confirmed the lower polymer concentration in the case of the softer-surface gel. Furthermore, the IR and the NR results showed that the softer surface does not collapse easily under a contact pressure up to 10 kPa. This indicates that despite the softer structure, the surfaces of hydrophobically molded gels are able to resist exudation at low pressures, which is presumably responsible for the good lubricity of these kinds of gels.

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