Applied Materials Today
Electroconductive multi-functional polypyrrole composites for biomedical applications
Graphical abstract
Introduction
Polymers have been perceived as highly promising materials for biomedical applications [1], [2], [3]. Because of their versatility, polymers have been utilized in bulk (e.g. scaffolds or hydrogels) or in colloidal form (e.g. nanoparticles or nanogels) to satisfy diverse medical needs [4,5]. Polymers are highly valuable because of their tunable mechanical properties and adjustable chemistry [6,7]. The choice of an optimal polymer for biomedical applications is determined by the physicochemical properties of the specific macromolecule. For example, a biologically stable polymer would be a better choice than a biodegradable polymer for the construction of an implantable medical device in which long-term stability is the primary concern. Likewise, a natural polymer is preferred over a synthetic polymer in a biomedical product in which cellular interactions are essential [1,8]. A major disadvantage of polymers in the construction of medical prosthetic devices to replace bone, heart or nerves is their lack of electrical conductivity. This is because the atoms in most polymer chains are joined together by strong covalent bonds with no freely movable electrons or ions. This feature renders most polymers effective as insulators but not conductors [9]. Nevertheless, some polymers may be tailored to be conductive when they are subjected to an electrical field. Properties such as the dielectric constant, dissipation factor and resistivity are used to define the electrical characteristics of a polymer [10].
Polymers that possess distinctive optoelectronic properties are termed inherently conducting polymers. The alternating single and double carbon-carbon bonds in these polymers have delocalized π-electrons and render them easily polarizable. Many studies have been conducted in this domain since Hegger, MacDiarmid and Shirakawa discovered the first inherently conducting polymer in 1977 [9,11,12]. Polypyrrole is the most widely used inherently conducting polymer in the biomedical field, as well as in other areas such as sensors, actuators, or energy generators (batteries and solar cells) because of its low cost and good electrical properties [13]. Another advantage of polypyrrole is its microbicidal and antioxidant properties, which have been attributed to the positive charges generated on the backbone during synthesis. There have been concerns over the toxicity of polypyrrole and related polypyrrole-based nanoparticles. The current trend is to combine polypyrrole with other organic or inorganic materials to improve the biocompatibility of the final product [14,15].
The present review outlines the structure and synthesis of polypyrrole, as well as the physical, chemical and mechanical properties of polypyrrole-containing nanocomposites. The antimicrobial and antioxidant properties and possible cytotoxicity of these nanocomposites are outlined and critiqued. The application of polypyrrole as biosensors, drug delivery systems, tissue engineering scaffolds and photo-thermal therapeutic agents is subsequently presented to spur further advances in this exciting field of research.
Section snippets
Structure and prepration
Polypyrrole is an electrically conducting polymer that is composed of heterocyclic monomers joined together. The molecular structure comprises five-membered pyrrole rings linked to each other at the 2- and 5-positions. It was first synthesized in 1963 by the pyrolysis of tetraiodopyrrole [16]. Nowadays, polypyrrole is usually prepared via electrochemical or chemical oxidation polymerization approaches (Fig. 1) [17,18].
Electrochemical polymerization of pyrrole is based on anodic oxidation of the
Physicochemical and mechanical properties
The electrical conductivity of polypyrrole is due to the ease with which electrons pass along and between the polymer backbones [35]. Polypyrroles possess a conjugated backbone, meaning that it is made up of a series of alternating single and double bonds with sp2-hybridized carbon atoms. Dopants play a key role in enhancing the electrical conductivity of polypyrrole. In its pristine (non-doped) state, polypyrrole has a band gap of 4 eV and is an electronic insulator. Once the polypyrrole is
Cytotoxicity and biocompatibility
Because polypyrrole and its composites have been used for biomedical applications extensively, it is logical to identify the factors that affect their biocompatibility so that these materials may be evaluated in clinical trials. Polypyrrole has been used for delivery of dexamethasone to the eye. The half maximal inhibitory concentration (IC50) of polypyrrole, which is a measure of its potency in inhibiting a specific biological function, 0.77 mM. This value indicates that the polypyrrole is
Antimicrobial activity
Polypyrrole is a conductive polymer with bactericidal activity. This activity is attributed to the strong electrostatic interactions between the positive charges along the polymer structure and the negatively charged bacterial cell wall [81]. These positive charges (NH+) are repeated every three to five units and are produced using dopants such as HCl, H2SO4, or HNO3. Polypyrroles can destroy bacterial cell wall, interact with the cell membranes of bacteria, disrupt the balance of proton
Drug delivery
Controlling the release rate of drugs loaded within polymeric carriers is an important challenge in drug delivery [111]. Polypyrrole has been employed as a component of drug delivery platforms because of its biocompatibility as well as its electrical properties. These properties enable polypyrrole to control the release of drugs and improve the overall efficacy of the drugs [112], [113], [114]. Drug release may be controlled by altering the degree of protonation of the polypyrrole to produce a
Biosensors
The requirements of biosensors include high selectivity, high sensitivity, stability, high accuracy, reusability, low-cost and user-friendliness. Biosensors are used for many biomedical applications, the most important of which is the early detection of biomarkers related to diseases or infections. Many studies have been conducted to identify useful materials for the fabrication of biosensors [217,218]. Conductive polymers with electrical conductivity are cost-effective, biocompatible and
Conclusions and future perspectives
In the present review, polypyrrole and its nanocomposites, their advantages and associated challenges were concisely reviewed. Issues including the preparation, physical, chemical, mechanical properties and biological properties, as well as biomedical applications were thoroughly discussed.
Tissue engineering strongly depends on the ability to expose cells that migrate into the damaged tissue to accurately defined biophysical, biochemical and biomechanical cues. The incorporation of
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgments
M.-A. Shahbazi acknowledges the financial support from the Academy of Finland (grant no. 317316). MR Hamblin was supported by US NIH grants R01AI050875 and R21AI121700.
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These authors equally contributed to this work.