(415i) Brushed Soft Contact Lenses for Reducing Lid-Wiper Sliding Friction

Brushed
Soft Contact Lenses for Reducing Lid-Wiper Sliding Friction

C.J. Radke¹, D.T.
Bregante¹, V.
Pang¹, T.J.
Dursch¹

¹ Chemical
and Biomolecular Engineering Department, University of California, Berkeley, CA

AIChE 2015: Session 08A04:
Polymer Thin Films and Interfaces

Minimal
sliding friction of soft contact lenses (SCLs) is critical to on-eye
comfort.^1,2 In spite of significant attention in recent years,
sufficient lens lubricity has not been achieved.² We investigate use
of strongly-hydrophilic polyelectrolyte brushes to reduce sliding friction of
SCL-hydrogel surfaces in the ?high? friction boundary lubrication regime that
is most likely the origin of wear discomfort. Anionic
poly(acrylic acid) (PAA,
435 kDa)
strands are physically entrapped in HEMA-based SCLs. Briefly,
aqueous-equilibrated SCLs were swollen in isopropanol (IPA), containing
dissolved PAA. Subsequently, PAA-containing SCLs were deswollen (i.e., solvent
exchanged) in aqueous phosphate-buffed saline (PBS, pH 7.4) to physically
entrap the polymer. Successful
brush attachment is confirmed by comparing surface forces measured by
colloidal-probe atomic force microscopy (AFM) on a SCL hydrogel before and
after brush treatment, and by fluorescence confocal laser-scanning microscopy
(FCLSM) of fluorescently-labeled polymers. By way of example, Figure 1 displays
a typical FCLSM image of an aqueous-equilibrated HEMA-based hydrogel
immediately following anionic FITC-labeled PAA brush attachment.  Fluorescence-intensity
profiles clearly confirm successful polymer entrapment, with no observable
FITC-PAA leaching even after 2 wks.

Aqueous
sliding friction forces are obtained using both colloidal-probe
AFM and the inclined-plane method (IPM). Figure
2 shows
typical
aqueous
sliding friction coefficients, CoFs, for a HEMA-based SCL and a PAA-brushed
HEMA-based SCL obtained from both AFM and the IPM. Several features are
salient. COF values are reduced significantly for the PAA-brushed HEMA-based
SCL compared to the unbrushed SCL. The CoF value for the HEMA-based SCL is 0.06
from both colloidal-probe AFM and the IPM, in in good agreement with literature
values.³ For the PAA-brushed HEMA-based SCL, however, the CoF value
measured by colloidal-probe AFM is nearly an order of magnitude lower than that
obtained from the IPM (i.e., 0.004 compared to 0.02). This
finding indicates that the IPM is unsuitable for obtaining low CoF values
(i.e., lower than 0.02). Our colloidal-probe AFM measurements reveal that
entrapped anionic PAA brushes in conventional HEMA-based SCL hydrogels reduce
lens sliding friction in the ?high? friction boundary lubrication regime by
over an order of magnitude.

1.
Brennan, NA. Contact lens-based correlates of soft lens wearing comfort. Optom
Vis Sci 2009;86:ARVO E-Abstract 90957.

2.
Nichols JJ, Jones L, Nelson JD, et al. The TFOS international workshop on
contact lens discomfort. Optom Vis Sci 2013;54:TFOS1-TFOS6.

3.
Peng C-C, Lim PS, Chong H, Dursch TJ, Radke CJ. Critical coefficient of
friction of in-vitro spoiled soft contact lenses. Optom Vis Sci 2014; ARVO
E-Abstract 4652.