Fabrication of hydrogel coatings by atmospheric-pressure plasma polymerization Function by structure and chemistry

Living organisms have developed numerous strategies to create function by internal structure at all levels of hierarchy, serving as role model and inspiration for the development of technical innovations, especially in the field of biomimetic materials and surfaces [1]. Considering the circular bioeconomy-driven sustainable management of natural resources, bio-based materials are becoming increasingly important for creating materials and surfaces, linked to the ecological way of thinking [2]. In these efforts, hydrogels - three-dimensional cross-linked polymeric networks - are among the most promising nature-derived materials applied in recent technologies for biomedical engineering, biotechnology, packaging, pharmaceutical science and agriculture. Polymeric hydrogel coatings provide various advantages to regular solid surfaces with superior properties with respect to strength, flexibility, biocompatibility, functionalities, permeability, and stimuli-responsiveness [3]. With their function as biomimetic material, synthetic hydrogels or hydrogel-forming natural polymers (e.g. collagen, alginate or agarose) are utilized to reproduce the filamentous nature and properties of extracellular matrix serving as scaffolds for 3D cell culture, artificial skin, actuators, biosensors, drug delivery vehicles and tissue-engineered products for cell matrices [4]. The enormous diversity provided by the design of multifunctional hydrogels is realized by combining hierarchical structuring inspired by nature with the variety of materials available by engineering are of utmost significance for the desired application. For the preparation of hydrogels, physical and chemical polymerization and crosslinking methods are traditionally involved. Here, we report on the fabrication of hydrogel coatings by using non-thermal gas discharges, referred to as plasma, a gaseous state in which free electrons and ionized atoms exist. Technological exploited plasmas, inspired by natural atmospheric discharges, offers unique capabilities of surface engineering including modifying selected surface properties and enabling surface functionalization along with the fabrication of special surface structures. Plasma polymerization resembles free-radical polymerization in which various active components of plasma, such as reactive species or radicals, play an important role in controlling for instance chemical and mechanical properties that determine the shrinking and swelling behavior of the obtained hydrogels. A novel approach to generate hydrogel coatings composed of 2-hydroxyethyl methacrylate (HEMA) and the tertiary amine methacrylate 2-(diethylamino)ethyl methacrylate (DEAEMA) copolymerized in 1:1 volume ratio through atmospheric-pressure plasma polymerization is presented. The plasma-based methodology for hydrogel synthesis includes three consecutive processes: 1. Plasma-based pre-treatment of the substrate leaving a layer of reactive surface radicals interacting with the liquid hydrogel monomer mixture that is subsequently deposited; 2. plasma-induced polymerization of the monomer units into polymer chains; and 3. cross-linking of the polymer chains into a polymer network while preserving the hydrogel structure. HEMA and DEAEMA were selected due to the presence of polymerizable structures (e.g. vinyl groups) and because of their capacities to undergo hydrogen bonding in the plasma-polymerized film leading to inter- and intramolecular cross-linking to maintain the integrity of the hydrogel, in addition to conventional cross-linking reactions during plasma polymerization, resulting in complex networked structures. Furthermore, by applying plasma technology, the route of chemical reactions is modified, which enables a high level of functionality as well as the formation of a hierarchical structure of the fabricated hydrogel coating. The microscopic image exhibits a thin (<2 µm) plasma-polymerized hydrogel film that represents a feather-like surface structure accompanied with a coloration resulting from a variation in the film’s thickness. This coating was synthesized by using an atmospheric-pressure plasma jet. By flowing argon through the discharge channel, the plasma generated is expelled into the surrounding medium, forming an effluent of a few millimeters. Based on this plasma jet configuration, which is geometrically confined to small dimensions in the micrometer to millimeter range, the polymerization is restricted to areas that are exposed to plasma. Hence, a hydrogel pattern was generated by a spatially resolved surface modification attributed to the filamentary nature of the argon discharge. The chemical and physical properties of the coating can be tuned by simply varying the mixture ratio of HEMA and DEAEMA. DEAEMA is pH responsive due to the presence of the tertiary amine, which can be protonated and deprotonated in acidic and alkaline conditions, respectively. Therefore, the surface charge or zeta potential, which affects the pH responsive behavior of the hydrogels, can be modulated by increasing the DEAMEA content in the copolymer. The wrinkles characteristics, such as wrinkle wavelength and wrinkle amplitude/height, depend on the total thickness of the hydrogel, which is strongly influenced by the mixture ratio and by the plasma process parameters. Wrinkle wavelength and amplitude are increasing with increasing content of HEMA. These chemical and physical properties of the hydrogels are preserved even after immersion in water for several days indicating the high stability. The simplicity of atmospheric-pressure plasma polymerization to fabricate stimuli-responsive hydrogels as “materials on demand” will certainly extend the applications of plasma-based hydrogels. One of the novel aspects enabled by the fabrication approach presented is the ability to prepare unique materials with tunable properties (i.e. chemical and physical) for a plethora of applications.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fabrication of hydrogel coatings by atmospheric-pressure plasma polymerization Function by structure and chemistry

Declaration of Competing Interest

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