Water Wettability of Hydrogels

19 Water Wettability of Hydrogels

Downloaded by UNIV OF MISSOURI COLUMBIA on June 7, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0031.ch019

FRANK J. HOLLY and MIGUEL F. REFOJO Eye Research Institute of Retina Foundation, 20 Staniford Street, Boston, Mass. 02114

The matrix of hydrogels consists of hydrophilic macromolecules which are crosslinked to form a three-dimensional network. Thus, a gel spontaneously imbibes water u n t i l the gel reaches equilibrium hydration. This equilibrium water content of the gel depends on crosslink density and on the amount and nature of hydrophilic sites i n the macratolecular matrix. With the exception of poly (hydroxyethyl methacrylate) [PHEMA], which i s not soluble i n water, the matrix of hydrogels i s usually made up of water-soluble polymers. At equilibrium hydration a large fraction of hydrogels consist of water. Hydrogels containing over 95% water are not uncarmon. This i s probably why i t has been implicitly assumed that hydrogels have a hydrop h i l i c surface. In other words, water i s expected to spread spontaneously over the surface of hydrogels, at least when the surface i s f u l l y hydrated. Past Work on Hydrogel Wettability Acrylic hydrogels are f a i r l y new and their wettability has only been examined recently. However, there are several reports i n the literature i n which the wettability of gelatin gels are considered. The f i r s t systematic investigation of the wettability of hydrated and air-dried gelatin gels appeares to have been made by Pchelin and Korotkina (]L). They found a water contact angle of 98° on hydrated gelatin and a contact angle of 115° on the a i r dried gelatin film. These authors also observed that the wetta b i l i t y of the gelatin gels depended on the polarity of the mate r i a l adjacent to the gel surface while the gel was being formed. Thus gelatin formed against paraffin or a i r was hydrophobic, while gelatin formed against a clean glass surface was hydrophili c . Garrett (2) also found gelatin to be hydrophilic when formed against a glass surface. Braudo and coworkers (3) studied the wettability of gelatin gels as a function of the gelatin concentration. The water content of the gel varied from 14% for the air-dried gels up to 86% 252

In Hydrogels for Medical and Related Applications; Andrade, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


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Water

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for the most dilute gels studied. They found that the water cont act angle on these gels varied frcm the surprisingly high value of 123° for the most hydrated gel to 87° for the dehydrated gels. These authors offer an explanation for the anomalous wettability of gelatin gels based on the preferred orientation of the water molecules i n a surface free-water layer of the gel. I t i s hard to see, however, why an oriented water layer having the characteris t i c s of the surface layer of pure water would not be wetted by water. I t i s suspected that the high, obtuse water contact angles were obtained because of surface dehydration and these values were further increased due to the roughness of the surface. More recently the agarose gel was the subject of an investig ation as to water wettability (4). I f the gel was formed under water or had been exposed to water for long periods of time, the surface was hydrophilic. On the other hand, i f the surface was formed while exposed to clean a i r , the surface was hydrophobic. When this hydrophobic layer was shaved off, the gel again had a hydrophilic surface. We have recently found (5) that gels made of the polymer PHEMA. appear to have a hydrophobic surface even when the gel i s f u l l y hydrated. The wettability of PHEMA. gels having equilibrium water contents between 31 and 42% has been found to be surpris ingly low: advancing contact angle values for water between 60 and 80° have been obtained by both the sessile-drop and the captive-bubble techniques. The receding contact angle values measured were found to be much lower but s t i l l larger than zero. Description of the Hydrogels Studied The wettability of several types of hydrogels, consisting of macrcmolecules more hydrophilic than PHEMA, was measured (Figure 1). Gels were prepared by simultaneous polymerization and crosslinking of purified monomers i n aqueous solutions i n molds con s i s t i n g of two glass plates separated by a silicone gasket (6). The copolymer of glyceryl methacrylate and methyl methacrylate [P(GMA/MMA) ] was obtained from Corneal Sciences, Inc. Boston, MA. P (GMA/MMA) was made by bulk polymerization. The gel surface was polished and then equilibrated with water. Poly (glyceryl methacrylate) [PGMA] forms gels that have twice the number of hydroxyl groups per monomer unit than does the PHEMA gel. Poly(hydroxyethyl acrylate) [PHEA] forms gels that are sim i l a r to PHEMA i n hydrophilic sites but this polymer does not have the hydrophobic, bulky methyl side group on the acrylic backbone. The preparation of the PHEMA gels was described previously (5). The crosslinking agents for PHEMA, PGMA, and PHEA are the diesters which are usually found accompanying the monoesters. Poly(acrylamide) [PAA] gels were prepared using Ν,Ν -methylene bisacrylamide as a crosslinking agent. In addition to these synthetic materials, a polysaccharide gel, agarose, consisting of D- and L-galactopyranose, was includ1



254

HYDROGELS FOR MEDICAL AND RELATED APPLICATIONS

ed i n the study along with another material not s t r i c t l y a hydro gel, poly (dimethyl siloxane) bulk-grafted with poly (vinyl pyrrolidone) [SID-g-PVP] (7), which was obtained from the firm E s s i l o r (Paris, France). The gels, with the exception of Ρ (GMA/MMA), were formed against glass, which was made hydrophobic for some of the PHEMA. gels by S i l i c l a d coating (5). The gels were thoroughly washed and stored i n d i s t i l l e d water (that was frequently changed) for at least one month prior to the wettability measurements. Under these conditions, the hydrophobic or hydrophilic nature of the glass surface i n contact with the gel during i t s formation did not seem to affect gel wettability. Method of Measurement of Wettability The contact angles were determined by a Ramè-Hart Goniometer. The contact angle of the sessile droplets was determined i n a closed chamber (Ramè-Hart Environmental Chamber for Goniometer). Clean a i r was circulated through two consecutive gas-washing bottles f i l l e d with water and the chamber i n a closed system by a v a r i s t a l t i c pump i n order to keep i t saturated with respect to water vapor. The captive-bubble technique was also employed occasionally using a chamber designed for this technique i n order to guard against the slight p o s s i b i l i t y of surface dehydration. As with PHEMA (5), this l a t t e r technique yielded contact angle values that were consistently a few degrees higher ( ! ) than the data obtained with the sessile-drop technique. Both the advancing and the receding contact angles were determined. The sessile droplets (or captive bubbles) were s l i g h t l y increased or decreased i n size after the contact with the gel surface had been established, and u n t i l the angle remained constant upon additional change i n volume. Thus, the contact angles measured were s t a t i c rather than dynamic values as the terms seem to imply. Gel Wettability as Characterized by Water Contact Angles Figure 2 displays both the advancing and receding contact angle values obtained with water on the various gels as a function of the equilibrium water content of the gels. As a reference, the average advancing and receding contact angle values for poly(methyl methacrylate) [PMMA] are also shown i n the figure. I t i s clear from the graph that only the agarose gel of very high water content (over 96%) i s completely wetted by water. The next most wettable gel i s PAA as expected from i t s chemical composition. The wettability of PGMA and PHEA are similar indicating that the additional hydroxyl group of PGMA just about cancels the hydrophobic effect of the methyl group absent i n PHEA. The wettability of the PHEMA gels, having considerably lower equilibrium hydration, i s widely variable, but a l l PHEMA gels are


19.

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Water Wettability

A N D REFOJO

H I

R I

I "k H


1.

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Hydrogels

1

I C

Ο

GEL

of

R

0

OR SOLI D PMMA

-CH

3

3

3

2.

PHEMA

-CH

3.

PGMA

-CH

4.

PHEA

-Η

-0CH CH 0H

5.

PAA

-Η

- NH

-OCHOHCH OH 0

9

0

9

6. P(GMA/MMA)

copolymer of 3. & 1.

7. SIL-g-PVP

PVP grafted

8. Agarose

polysaccharide (galactopyranose)

Figure

onto s i l i c o n e

1. Chemical composition of hydrogels investigated

T=25°C

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ο ζ

Water Wettability of Hydrogels

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