Characterization of the inner surface of tubular substrates by contact angle measurement

H. Steinhauser and B. Jamen* Institute of Physical Chemistry, University of Cologne, Luxemburger Strasse, 116, D-5000 Cologne 4 1, FRG (Received 23 February 1988; revised 30 June 1988; accepted 10 July 1988)

A new method for the direct measurement of the contact angle on the inner surface of tubular substrates is described. It is shown that the measured contact angles are in good accordance with those obtained on flat surfaces.

Keywords: Surface characterization, contact angle, tubular substrates

Measurement of the contact angle is a fast and well- established method to characterize surfaces and follow surface modifications’. Contact angle measurements are frequently used for surface characterization and estimation of surface free energies of flat as well as convex surfaces especially in the field of biomedical materials*. However, many of the biomaterials used in practice are circular tubings, such as catheters, vascular protheses, shunts or fluid drainages, where the concave inner surface is of interest. As standard measurements of the contact angle are not applicable to these substrates, flat polymer films are very often made from the original material, for example by solvent casting. The contact angles obtained on such surfaces are, however, incorrect, as influences due to the fabrication process of the tubing cannot be estimated. One method of performing contact angle measurements on the inner surface of circular tubings is to cut them longitudinally and fix them on a flat carrier by pressing3. This manipulation always bears the danger of contaminating the surface either by touching it accidentally, or during the pressing while the glue hardens or by the solvent in the glue. To overcome these problems, we have developed a new method for contact angle measurement directly on the inner surface of circular tubings using a conventional goniometer and sessile drop technique.

Polymer tubings (d = 4 mm) were cut longitudinally (axially) and the liquid drop was placed in the centre of the half shell with a microsyringe (dropvolume: 2.0pl).Thedrop was adjusted in a way so that the two contact points in the projection were at the same level. By means of a Lorentzen and Wettre contact angle goniometer, the height and the

*Present address: Hygiene Institute. University of Cologne. Goldenfelsstrasse, 19-Z 1, D-5000 Cologne, FAG Correspondence to Dr H. Steinhauser.

Figure 1 Determination of the conract angle on a cowed surface: d = tube diameter, h = height of the drop, b = breadth of the drop and conract angle e=o,+o,

breadth of the drop were measured as shown in Figure 1. The contact anglewas calculated according to Equation 1.

0 = 0, + O2 = arcsin (b/d) + arctan (2/1/b) (I)

with d is the diameter of the tubing in the projection, h is the height of the drop and b is the breadth of the drop.

Polyetherurethane (PUR), chlorinated polyethylene (CPE) and silicone rubber tubings were used after extraction in ethanol or cyclohexane, respectively. To prove the validity of our method in the investigation of surface modifications, the tubings were grafted with the hydrophilic monomer vinylmethylacetamide (VIMA) by glow discharge grafting3.

At least ten measurements were performed per sample. The contact angles obtained on the curved surfaces

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Biomaterkls 1989, Vol 10 April 213

Table 1 Comparison of the water contact angles of polymer tubing obtained on flat and curved surfaces

Polymer Water contact angle Water contact angle (flat surface) (curved surface)

(B’ + SD) (So + SD)

PUR 72.0 I 2.1 74.2 + 2.8

CPE 88.0 It 3.1 85.5 f 2.3 SR 115.2 -1. 3.8 1 1 1 .O + 4.6 PUBg-VIMA 41.1 f 3.2 47.2 +_ 3.4 CPE-g VIMA 33.8 + 2.8 36.7 I 3.8 SR~-VIMA 43.4 t 2.3 45.4 t 2.7

SD = standard deviation.

were compared with the contact angles measured on flattened surfaces glued to a fiat carrier.

Comparison of the contact angles (Table 7) shows, that the absolute values of the angfes differ, probably because the gravitation force does not act perpendicularly on the contact point. However, the differences are of the same order of magnitude as the standard deviations, so that the

inaccuracy of the method seems to be tolerable. In conclusion, we believe that the described method is a useful tool to characterize the inner surface of tubings and to follow surface modifications thereon.

ACKNOWLEDGEMENT

We thank Mrs U. Treitz for her skilful support.

REFERENCES

1 Andrade, J.. Smith, L. and Gregonis, D., The contact angle and interface energetics, in Surface and InterfacialAspects of Biomedical Polymers (Ed. J. Andrade), 1, Plenum Press, New York and London, 1985, pp 249-292

2 Mad&fan, M.. Hdden. B. and Fang D.. A new method for wetting angle measurement, International Eyecare 1986.2.45-49

3 Jansen, B. and Steinhauser, H.. Plasma modification of the inner surface of polymer tubes for the improvement of anticoagulant properties, Advances in Biometerials 1986, 6, 207-2 12

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