Polymer materials offer several advantages as supports of biosensing platforms in terms of flexibility, weight, conformability, portability, cost, disposability and scope for integration. The present study reviews the field of electrochemical biosensors fabricated on modified plastics and polymers, focusing the attention, in the first part, on modified conducting polymers to improve sensitivity, selectivity, biocompatibility and mechanical properties, whereas the second part is dedicated to modified “environmentally friendly” polymers to improve the electrical properties. These ecofriendly polymers are divided into three main classes: bioplastics made from natural sources, biodegradable plastics made from traditional petrochemicals and eco/recycled plastics, which are made from recycled plastic materials rather than from raw petrochemicals. Finally, flexible and wearable lab-on-a-chip (LOC) biosensing devices, based on plastic supports, are also discussed. This review is timely due to the significant advances achieved over the last few years in the area of electrochemical biosensors based on modified polymers and aims to direct the readers to emerging trends in this field.

Composite i-PP/PEDOT films made of isotactic polypropylene (i-PP), which is frequently used for the fabrication of implantable medical devices for internal use, and chemically synthesized poly(3,4-ethylendioxythiophene) (PEDOT) nanoparticles, which are electroactive and biocompatible, have been prepared and used to detect bacterial metabolism. After chemical and morphological characterization, the properties (interfacial, mechanical, thermal and electrochemical) and biocompatibility of i-PP/PEDOT have been examined. Besides, carbon screen-printed electrodes coated with i-PP/PEDOT have been found to detect the growth of Gram-positive and Gram-negative bacteria through the oxidation of nicotinamide adenine dinucleotide (NADH), which comes from the bacteria metabolism (i.e. the respiration). Thus, as outer bacterial membranes are permeable to cytosolic NADH, this metabolite has been found to be an appropriated target for the detection of bacterial proliferation. In addition, the sensor does not respond towards eukaryotic cells. This is because the major NADH pool in eukaryotic cells is located at the mitochondria and, therefore, the concentration of in the medium is not high enough to be detected since the inner mitochondrial membrane is impermeable to NADH or NAD+.

Amphiphilic copolymers consisting of an all conjugated polythiophene backbone and sparsely attached oligo-ε-caprolactone side chains have been prepared by anodic electropolymerization of hydroxymethyl-3,4-ethylenedioxythiophene (HMeEDOT) with a thiophene-ended oligo-ε-caprolactone macromonomer (Th-PCL), obtained by ring opening polymerization of ε-caprolactone with thiophene methanol. The random copolymers, obtained starting from two different molar ratios of the co-monomers in the feed (HMeEDOT : Th-PCL of 80 : 20 and 60 : 40), and the homopolymer (PHMeEDOT) were synthesized by using three different working electrodes. After structural characterization by FTIR, the electrochemical, morphological and surface properties of the obtained copolymers were examined, and the results evidenced a dependence on both the working electrode and the composition in the feed. In order to evaluate the opportunities in the copolymers's further bioapplications, biodegradability, cytotoxicity and cell proliferation investigations were carried out. By combining the results of electrochemical characterization with those of biocompatibility and dopamine sensing capability tests, it was concluded that a co-monomer feed ratio of 80 : 20 could be the optimal choice for the potential use of these amphiphilic copolymers in sensing devices. All in all, this study shows the benefit of a designed “hairy-rod” conjugated polymeric architecture which, via their side chain nature (biocompatible/biodegradable, hydrophobic, oligomeric) and their grafting density, enabled the synthesis of a material for targeted application.

New amphiphilic poly(ethylene glycol) (PEG)-grafted random intrinsically conducting copolymers which combine three different functionalities have been engineered, prepared and characterized. Specifically, these “rod-coil” type copolymers bear conducting polypyrrole (PPy) and poly(Schiff base) (PSB) sequences randomly distributed in their backbones; hydrophilic grafted side chains consisting of well-defined PEG chains are attached to the PSB units. Basically, the synthesis of the copolymers is conducted sequentially by employing the “macromonomer” technique via electrochemical co-polymerization of a bis (pyrrole) benzoic Schiff base-containing PEG macromonomer with pyrrole monomers. After investigation of the chemical and electrochemical properties of the synthesized copolymers, their advantages of multi-functionality in terms of biomedical applications have been demonstrated. More specifically, the conjugated PPy and PSB sequences enabled the grafted copolymers to exhibit great ability to catalyse the oxidation of serotonin, an important neurotransmitter found in blood platelets and in the central nervous systems of animals and humans. On the other hand, the enhanced biocompatibility in comparison with bare PPy is due to the presence of PEG side chains, while bacteriostatic activity against both Gram-negative and Gram-positive bacteria is imparted by the synergistic combination of the polycationic character of the PPy main chain with the benzoic Schiff base functional groups and with the bent-shaped architecture of the facially amphiphilic PSB sequences, respectively. Accordingly, these grafted copolymers are promising materials for developing implantable electrodes for serotonin detection which present the abovementioned characteristics.

Development of smart functionalized materials for tissue engineering has attracted significant attention in recent years. In this work we have functionalized a free-standing film of isotactic polypropylene (i-PP), a synthetic polymer that is typically used for biomedical applications (e.g. fabrication of implants), for engineering a 3D all-polymer flexible interface that enhances cell proliferation by a factor of ca. three. A hierarchical construction process consisting of three steps was engineered as follows: (1) functionalization of i-PP by applying a plasma treatment, resulting in i-PP_f; (2) i-PP_f surface coating with a layer of polyhydroxymethy-3,4-ethylenedioxythiophene nanoparticles (PHMeEDOT NPs) by in situ chemical oxidative polymerization of HMeEDOT; and (3) deposition on the previously activated and PHMeEDOT NPs coated i-PP film (i-PP_f/NP) of a graft conjugated copolymer, having a poly(3,4-ethylenedioxythiophene) (PEDOT) backbone, and randomly distributed short poly(ε-caprolactone) (PCL) side chains (PEDOT-g-PCL), as a coating layer of ∼9 μm in thickness. The properties of the resulting bioplatform, which can be defined as a robust macroscopic composite coated with a “molecular composite”, were investigated in detail, and both adhesion and proliferation of two human cell lines have been evaluated, as well. The results demonstrate that the incorporation of the PEDOT-g-PCL layer significantly improves cell attachment and cell growth not only when compared to i-PP but also with respect to the same platform coated with only PEDOT, constructed in a similar manner, as a control.