MATERIALS
Expanding functionality
In the field of medical device development there are a number of factors generally recognised as being important for success. Among these are the biocompatibility, sterility, reliability and adaptability of materials to their surroundings. Without a suitable approach to these issues, the majority of medical devices will not be as successful as they could be. Biocompatibility of materials, in particular, is a critical factor in the development and application of permanent and temporary implants and other devices such as catheters and tubes that are to be used in and around the body. Coating technology is the obvious and ideal solution for separating the bulk properties of a material or device from direct interaction with its surroundings. The independent modification of surface and bulk properties widens the range of features that can be incorporated into products. Bulk properties are responsible for characteristics such as mechanical strength. A suitable coating will enhance the interaction of the device with its surroundings. For example, it will provide drug-elusion (stents), anti-fouling and antibacterial properties, and a hydrophobic self-cleaning surface, referred to as lotus coating.1 The lotus effect in material science is the observed self-cleaning property found with lotus plants. A coating with this effect will make surfaces self-cleaning and will decrease the need for active cleaning of the subsequent surface; it may even enhance the sterility of surfaces.
Recently there have been some interesting developments in materials and coatings based on organic and inorganic components, which are responsible for current state-of-the-art devices. Examples include coatings for stents that provide multiple therapeutic effects in thinner layers and coatings with better adhesion to device surfaces. The future holds the promise of even greater functionality for medical coatings.
Polymer brushes
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The so-called brush coatings are a recently developed coating technology, which will lead to new functionalities and possibilities.2 These monolayer coatings of modified polymer chains are based on chemically bonded molecular “hairs.” Figure 1 [8] shows a schematic representation of a brush coating. By selecting the right types of molecular hairs the surface of a material can be modified (Figure 2 [9]). This procedure makes it possible to create biocontact properties of surfaces with a coating that is only a few nanometers thick. Coatings such as these can create surfaces with specific features.
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Figure 2. (click to enlarge) [9] Water droplet on surface modified with a hydrophobic brush coating.
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Adhesion
Antifouling. One of these surface properties is antifouling and it has a range of applications. It can improve the cleanliness of surfaces; increase biocompatibility; prevent tissue and bacterial adhesion on ophthalmic, urological, intrauterine and contraceptive devices, and on catheters and stents; and provide a barrier on medical textiles and orthopaedic pins.
Adhesion improvement. Instead of preventing adhesion, it is also possible to improve adhesion. More options can be created for certain molecules, bacteria and cells by decreasing adhesion of nonspecific compounds while stimulating specific interaction. Intelligently combining a polymer brush that prevents adhesion with brush hairs modified with antibodies, promoters or other biomolecules can produce a range of sensor-based applications because of the optimised specific adhesion that is available. Possibilities here are specific disease or drug detection when suitable hairs are combined with the antifouling brush.
Other factors for functionality
In addition to types of hair, there are several other factors that are relevant to the functionality of these polymer-brush coatings. An overly dense brush can prove detrimental to its functionality. If the graft density of the polymers is high enough, the intermolecular interaction results in the grafted polymers being stretched. This will make the repellent action of the brush better. If the polymers are uniformly stretched and all chain ends are located at the edge there will be hardly any, or no, adherence to the brush for an extended period of time.
This polymer-brush technology makes it possible to modify biocontact properties with coatings of nanometer thickness. In addition, this technology can enable the incorporation of a range of features into thin coatings, for example, specific adhesion and nonadhesion with a coating of less than 50 nanometers.
Contact angle
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Figure 3. (click to enlarge) [10] Light microscope and confocal microscope image of structured coating.
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Contact angle is another important characteristic of a coating and can be a decisive factor. Suitable control and modification of the contact angle are essential and it is fortunate that there is a range of technologies available for this. In general these are based on two factors: chemical modification or topology modification. In the former, fluor-based brushes (molecular compounds containing fluor atoms) are a suitable option for increasing contact angle. Unfortunately, with chemical modification it is not possible to obtain a contact angle of more than 130° and in some cases a higher contact angle is advantageous. Because air has the highest possible contact angle (180°), it could be desirable for the contact between liquid and air to be increased. Thus, reducing the contact surface between liquid and substrate will create a higher contact angle. When a surface has a regular and well-controlled surface roughness (preferably on a micrometer or nanometer scale), it is possible to create “super” hydrophobic or hydrophilic surfaces (Figure 3 [10]). Laboratory tests described in the available literature have shown that when the right coating and topology are used, contact angles close to 180° are possible.3
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Figure 4. (click to enlarge) [11] Increasing the contact angle by introducing surface roughness.
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Nanocomposite technology is particularly suitable for modifying surface topology. Generally speaking, a nanocomposite is not used as a coating, however, it can add extra functionalities to surface finishes. In cases where nanoparticles are chemically modified in a suitable manner, it is possible to migrate the particles to the coating surface in a controlled manner to give in the desired surface chemistry and topology of the applied coating (Figure 4 [11]). “Super” hydrophobic surfaces, that is surfaces with a contact angle of >50°, can be obtained, and functional reactive groups and material adhesion properties can be introduced onto a surface.
To obtain this (nano)structuring effect, two important contradictory properties must be taken into account. The (organically modified) clay particles, which are used to obtain nanocomposites, must be dispersed in the coating medium on a nanoscale;4,5 this can only be achieved by making the particles compatible with the coating system. Moreover, the part-icles need to migrate to the coating surface to generate a surface roughness and hydrophobicity; this requires a certain degree of incompatibility. In other words, a delicate balance of modification of the nanoparticles is necessary to obtain this effect. If a suitable topology and chemical modification is obtained by using the correct modification and particle concentration, these surfaces will have a high contact angle and will display the lotus effect described earlier.1
Other nano properties
In addition to the high contact angle, more properties are possible using nanocomposite coatings. Aligning the modified nanoparticles horizontally (instead of vertically as is required for the lotus effect) can significantly increase surface-barrier properties. It has also been demonstrated in this author’s laboratory that the water barrier of a sol-gel coating can be increased 15 fold, which can expand the range of applications for this type of coating. This property is interesting for medication packaging, tubing and applications where water permeation may be a problem.
Future prospects
Accurate control over surface chemistry and topology is extremely important to the optimisation of the interactions with biological surroundings. The current range of surface modification technologies is advancing annually and new technologies such as polymer brushes and nanocomposite coatings are being incorporated into medical devices. The use of advanced coatings increases the range of material combinations, which will lead to new and innovative device solutions. Characteristics such as controlled release, autocleaning surfaces, improved barrier properties and enhanced specific biointeractions and combinations are within reach as a result of ever-advancing medical coating technologies.
1. http://en.wikipedia.org/wiki/Lotus_effect [12]
2. A. Roosjen et al., “Microbial Adhesion to Poly(Ethylene Oxide) Brushes: Influence of Polymer Chain Length and Temperature,” Langmuir, 20, 25, 10949–10955, 2004.
3. L. Gao and T.J. McCarthy, “The ‘Lotus Effect’ Explained: Two Reasons Why Two Length Scales of Topography Are Important,” Langmuir, 22, 7, 2966–2967, 2006.
4. http://en.wikipedia.org/wiki/Nanocomposite [13]
5. R.G.J.C. Heijkants and L.F. Batenburg, “Next Generation Polymer Nanoconmposites,” Medical Device Technology, 7, 8, 10–12, 2006.
Ralf G.J.C. Heijkants is a Project Leader in Innovative Materials, TNO Science and Industry, PO Box 6235, 5600HE Eindhoven, The Netherlands.
tel. +31 40 2650467 e-mail: ralf.heijkants@tno.nl [14] www.tno.nl [15]