ABSTRACT

Regarding swelling, the main difference between cross-linked and non-cross-linked polymers is that with the former, the entry of liquid cannot separate the chains since they are covalently bonded, while with the latter, the entry of liquid can untangle the chains and separate them, as the forces keeping them bonded are of physical origin and less magnitude. There is, however, no radical difference between them, as in most cases the cross-linking in the hydropolymers is not only due to covalent bonds (σ bonds), typical of any cross-linked material, but also due to intermolecular van der Waals forces and the hydrogen bonds. There are also other types of interactions, such as electrostatic forces, both attractive and repellent, intermolecular bonds of hydrophobic components and ionic interactions. Hydropolymers have a set of general characteristics of which the most significant are ∑ They are of a hydrophilic nature due to the presence of water-soluble groups in the structure (-OH, -COOH, -CONH2, -CONH, -HSO3). ∑ They are insoluble in water due to the existence of a three-dimensional polymer network that gives the structure consistency. ∑ They have a smooth elastic consistency that is easily adaptable to human tissue. This is determined by the initial hydrophilic monomer and the low density of the polymer cross-linking. ∑ They swell up in water and considerably increase their volume until a physical-chemical balance is reached, while, in principle, keeping their original geometric proportions. ∑ They are thixotropic materials; that is, they become more fluid on being agitated or when subjected to alternate mechanical stress, and become more solid when they are in a state of rest

(e.g., a typical effect that can be seen in plastic paint). On the other hand, since hydropolymers were introduced into the field of Health Sciences (mainly Biology and Medicine), they have been shown to have a great potential as biomaterials, due to their good response when in contact with biological tissue. This usually leads to devices that are suitably biocompatible and endorses their use. This remarkable feature is because the physical properties of hydrogels are more similar to those of living tissue than any other

kind of synthetic biomaterial, particularly with regard to their relatively high water content, their soft, elastic consistency and their low surface tension. Of the typical polymers possessing these properties that are finding the largest number of applications in medical device and application development, it is worth mentioning the following: poly(vinyl alcohols) or PVAs, poly(acrylic acids) or PAAs, silicon hydrogels, gelatine hydrogels, poly(glutamic acid), polyacrylamides, poly(HEMA), N-vinylipyrolidone among others. The following section contains a brief summary of the existing applications and the most recent proposals for use for these materials as integral parts of medical devices, both passive and active. 12.2   Potential for BiodevicesThe medical applications of these materials can be divided into active or passive according to their role in a specific device. Their passive applications in “passive devices” are based on the excellent properties of these materials (in their hydrated phase) when in contact with human tissue. Their active applications in “active devices” are usually based on making use of the super absorbent properties of these materials or based on the changes in geometry that occur when in contact with water so that hydroactive sensors and actuators can be produced. We shall now examine some of the main applications of some of the most highly developed materials to date. After undergoing in vitro and animal in vivo testing and meeting the requirements for official approval, in some cases their commercialisation is subject to their attaining the goals described at the end of this chapter. 12.2.1  Passive Devices 

Contact lensesThe good hydration capability of hydrogels and the possibility of producing devices with mechanical properties similar to those of many body tissues have led to hydrogels becoming standard materials for medical devices. A specific example is contact lenses where silicone hydrogel lenses are in general use. Their porosity allows oxygen to pass through specifically produced membranes, for which reason they are being used for disposable contact lenses

(with a useful life that ranges from 12 to 14 h up to one month) with ever better optical and ergonomic properties, compared to more traditional materials like PMMA and others.