**4. Effect of aqueous environments on plasma polymers**

For biomedical applications, the effect of water on the plasma polymer films is of particular importance.

Immersed in a solvent, plasma polymers can be subject to numerous processes such as:


However, not many studies focus on the physic-chemical changes that happen to the plasma polymer films after exposure to aqueous environments.

Plasma polymer stability behavior depends on the type of the polymer. Muir et al. [72] studied the penetration of water into the films and characterized the swelling of allylamine (Aam) and heptylamine (HA) plasma polymers. When immersed in water, the plasma polymerized Aam film (ppAam) was found to swell by 5% and to contain 3% water whereas the ppHA film did not appear to swell but contained 5% water. The swelling characteristics of other plasma polymers have also been reported [73–75].

Moreover, the degree of swelling strongly depends on the plasma process parameters. Zhang et al. [73], demonstrated that ppAam deposited at 20 W only shows a small degree of swelling while ppAam deposited at 5 W shows a large degree of swelling. This is due to the fact that at low powers the plasma polymer contains a large number of oligomers which are not covalently bound to the film; these oligomers can thus be readily extracted in the solvent. In fact, when studying the morphology of ppHA, Vasilev et al. [76] found that pores of several nanometers in diameter were formed after ppHA has been immersed in water for 24 h (see **Figure 2**). And the dimension of the pores was found to depend on the deposition conditions with larger pores obtained at lower powers (see **Figure 3**). This was attributed to oligomer water extraction after low molecular weight fragments were detected in the water. This results in the formation of gaps in the film and leads to ruptures of the polymer chains thus forming the observed porosity.

Förch et al. [77] found that for ppAam, the roughness of the polymer film increased from 0.85 to 1.26 nm after soaking in water which was attributed to the swelling of the film in water;

**Figure 2.** AFM topographic images of HA plasma polymer films deposited with a power of 20 W: (a) as deposited, (b) after immersion in water for 24 h.

**Figure 3.** AFM images of HA plasma polymer films deposited with a power of 50 W: (a) as deposited or (b) after immersion in water for 24 h.

whereas, Tarasova et al. [78] used XPS to study the changes in surface chemistry of ppAam and ppHA after immersion in water for up to 24 h. Results were similar to the ones obtained after these plasma polymers were stored in air, both undergoing rapid oxidation; amine and imine groups were converted to amides with an increase of C─O and C═O groups.

In order to improve plasma polymers stability, studies on the interaction of plasma polymer with the aqueous environment as a function of plasma deposition parameters have been conducted. Optimizing these parameters was shown to be very important in reducing the induced changes [77, 79]. Moreover, substrate pretreatment for cleaning or activation was also shown to prevent the delamination of the polymer film [77].

However, enhancement of plasma polymer stability is still insufficiently studied and more effort is still needed for a precise stability evaluation and quantification.
