Maria M. Pérez-Madrigal

Powering the future: application of cellulose-based materials for supercapacitors

In recent years, significant research has aimed at developing environmentally friendly supercapacitors by introducing biopolymeric materials, such as polysaccharides or proteins. In addition to the sustainability and recyclability of such novel energy storage devices, these polymers also provide flexibility, lightweight nature and stable cycling performance, which are of tremendous importance for applications related to wearable electronics. Among the different sustainable natural polymers, cellulose deserves special consideration since it is the most abundant and is extensively recycled. Consequently, research on electrically active cellulose-based supercapacitors has noticeably increased since 2012, which makes this review on the field timely. Specifically, recent advances in preparing high performance cellulose supercapacitors are summarized. Moreover, the key roles of cellulose in improving the specific capacitance and cycling stability of cellulose-based devices are compiled to offer important fundamental guidelines for designing the next generation of all-cellulose energy storage devices that are to come. Finally, challenges and perspectives in this exciting area of study are also discussed.

Towards sustainable solid-state supercapacitors: electroactive conducting polymers combined with biohydrogels

Solid-state organic electrochemical supercapacitors (OESCs) have been fabricated using poly(3,4-ethylenedioxythiophene) (PEDOT) electrodes, a biohydrogel as electrolyte system, and polyaniline fibers as redox additive. The effectivity of sodium alginate, κ-carrageenan, chitosan and gelatin hydrogels as electrolytic media has been evaluated considering different criteria. Results indicate that κ-carrageenan-based hydrogel is the most suitable to perform as electrolyte due to the appropriate combination of properties: mechanical stability, ease of preparation, lack of water leaking, and good medium for the electrochemical response of PEDOT electrodes. Cyclic voltammetry and galvanostatic charge–discharge assays indicate that OESCs based on PEDOT electrodes and κ-carrageenan hydrogel as electrolyte exhibits a good supercapacitor response in terms of specific capacitance, cycling stability, small leakage current and low self-discharging tendency. On the basis of these good properties, four OESC devices were assembled in series and used to power a red LED, confirming that, in addition to advantageous characteristics (e.g. elimination of liquid leaking and enhancement of the device compactness), the designed biohydrogel-containing OESC exhibits potential for practical applications. On the other hand, preliminary assays have been performed loading the κ-carrageenan hydrogel with polyaniline nanofibers, which act as a redox additive. OESC devices prepared using such loaded biohydrogel have been found to be very promising and, therefore, future work is oriented towards the improvement of their design.

Bioactive and electroactive response of flexible polythiophene:polyester nanomembranes for tissue engineering

Properties of free-standing nanomembranes prepared by blending poly(3-thiophene methyl acetate) and poly(tetramethylene succinate), a soluble polythiophene derivative and a biodegradable polyester, respectively, have been examined. The outstanding flexibility and robustness of the nanomembranes floating in ethanol have been demonstrated through aspiration in pipette/release/shape recovery cycles, which were repeated without cracking the film. The blend retains the electrochemical properties (i.e. oxidation and reduction processes) of the individual conducting polymer in both physiological and organic environments. Hydrolytic and enzymatic degradation assays show that the degradation of the polyester domains produces the detachment of the conducting polymer domains. The cellular viability, which has been studied using four different cellular lines, is significantly higher for the blend than for the polyester, indicating that the former material is a potential bioactive platform for tissue engineering. Finally, the electrobioactivity of the individual materials and the blend coated with cellular monolayers shows some dependence on the cellular line.

Biodegradable free-standing nanomembranes of conducting polymer:polyester blends as bioactive platforms for tissue engineering

The present study reports the fabrication of free-standing nanomembranes with semiconducting and biodegradable properties. Nanomembranes have been prepared by spin-coating mixtures of a semiconducting polythiophene derivative, poly(3-thiophene methyl acetate), and a biodegradable polyester, poly(tetramethylene succinate). Both the roughness and thickness of the nanomembranes, which ranged from 3 to 20 nm and from 20 to 80 nm, respectively, were precisely controlled through the spin-coater speed and the solvent evaporation properties. Nanomembranes made of conducting polymer/polyester blends, which are able to retain the properties of the individual polymers, are stable in air and in ethanol solution for more than one year, facilitating their manipulation. Enzymatic degradation essays indicated that the ultra-thin films are biodegradable due to the presence of the aliphatic polyester. Interestingly, adhesion and proliferation assays with epithelial cells revealed that the behavior of the blend as cellular matrix is superior to that of the two individual polymers, validating the use of the nanomembranes as bioactive substrates for tissue regeneration.

Insulating and semiconducting polymeric free-standing nanomembranes with biomedical applications

In recent decades, polymers have experienced a radical evolution: from being used as inexpensive materials in the manufacturing of simple appliances to be designed as nanostructured devices with important applications in many leading fields, such as biomedicine at the nanoscale. Within this context, polymeric free-standing nanomembranes - self-supported quasi-2D structures with a thickness ranging from ∼10 to a few hundreds of nanometers and an aspect ratio of size and thickness greater than 106- are emerging as versatile elements for applications as varied as overlapping therapy, burn wound infection treatment, antimicrobial platforms, scaffolds for tissue engineering, drug-loading and delivery systems, biosensors, etc. Although at first, a little over a decade ago, materials for the fabrication of free-standing nanosheets were limited to biopolymers and insulating polymers that were biodegradable, during the last five years the use of electroactive conducting polymers has been attracting much attention because of their extraordinary advantages in the biomedical field. In this context, a systematic review of current research on polymeric free-standing nanomembranes for biomedical applications is presented. Moreover, further discussion on the future developments of some of these exciting areas of study and their principal challenges is presented in the conclusion section.

Thermoplastic Polyurethane:Polythiophene Nanomembranes for Biomedical and Biotechnological Applications

Nanomembranes have been prepared by spin-coating mixtures of a polythiophene (P3TMA) derivative and thermoplastic polyurethane (TPU) using 20:80, 40:60, and 60:40 TPU:P3TMA weight ratios. After structural, topographical, electrochemical, and thermal characterization, properties typically related with biomedical applications have been investigated: swelling, resistance to both hydrolytic and enzymatic degradation, biocompatibility, and adsorption of type I collagen, which is an extra cellular matrix protein that binds fibronectin favoring cell adhesion processes. The swelling ability and the hydrolytic and enzymatic degradability of TPU:P3TMA membranes increases with the concentration of P3TMA. Moreover, the degradation of the blends is considerably promoted by the presence of enzymes in the hydrolytic medium, TPU:P3TMA blends behaving as biodegradable materials. On the other hand, TPU:P3TMA nanomembranes behave as bioactive platforms stimulating cell adhesion and, especially, cell viability. Type I collagen adsorption largely depends on the substrate employed to support the nanomembrane, whereas it is practically independent of the chemical nature of the polymeric material used to fabricate the nanomembrane. However, detailed microscopy study of the morphology and topography of adsorbed collagen evidence the formation of different organizations, which range from fibrils to pseudoregular honeycomb networks depending on the composition of the nanomembrane that is in contact with the protein. Scaffolds made of electroactive TPU:P3TMA nanomembranes are potential candidates for tissue engineering biomedical applications.

Self‐Assembly of Tetraphenylalanine Peptides

Three different tetraphenylalanine (FFFF) based peptides that differ at the N- and C-termini have been synthesized by using standard procedures to study their ability to form different nanoassemblies under a variety of conditions. The FFFF peptide assembles into nanotubes that show more structural imperfections at the surface than those formed by the diphenylalanine (FF) peptide under the same conditions. Periodic DFT calculations (M06L functional) were used to propose a model that consists of three FFFF molecules defining a ring through head-to-tail NH3(+)⋅⋅⋅(-)OOC interactions, which in turn stack to produce deformed channels with internal diameters between 12 and 16 Å. Depending on the experimental conditions used for the peptide incubation, N-fluorenylmethoxycarbonyl (Fmoc) protected FFFF self-assembles into a variety of polymorphs: ultra-thin nanoplates, fibrils, and star-like submicrometric aggregates. DFT calculations indicate that Fmoc-FFFF prefers a parallel rather than an antiparallel β-sheet assembly. Finally, coexisting multiple assemblies (up to three) were observed for Fmoc-FFFF-OBzl (OBzl = benzyl ester), which incorporates aromatic protecting groups at the two peptide terminals. This unusual and noticeable feature is attributed to the fact that the assemblies obtained by combining the Fmoc and OBzl groups contained in the peptide are isoenergetic.

Microfibres of conducting polythiophene and biodegradable poly(ester urea) for scaffolds

Hybrid scaffolds constituted of a mixture of conducting and biodegradable polymers are obtained by the electrospinning technique. Specifically, poly(3-thiophene methyl acetate) (P3TMA) and a copolymer derived from L-leucine, which bears ester, urea and amide groups (PEU-co-PEA), have been employed. Both polymers were selected because of their intrinsic properties and their high solubility in organic solvents. The biodegradable polymer renders continuous and homogeneous microfibers under most of the electrospinning conditions tested, appearing to be an ideal carrier for the polythiophene derivative. A spontaneous phase separation has been observed for concentrated solutions of PEU-co-PEA and P3TMA in chloroform–methanol mixtures. An enriched dense phase results on the conducting polymer and can be successfully electrospun, giving rise to scaffolds with up to 90 wt% of P3TMA. Morphological observations have indicated that continuous and regular microfibers are attained despite the high conducting polymer content. P3TMA presents a high doping level and leads to stable electrospun scaffolds by the simple addition of a low percentage of a high molecular weight carrier. The resulting scaffolds are practically amorphous and thermally stable, also presenting a pronounced electrochemical response and being electrochemically active. Thus, the formation of polarons and bipolarons at specific positions, the ability to exchange charge reversibly and the electrical stability of hybrid PEU-co-PEA/P3TMA electrospun scaffolds and P3TMA alone are practically the same.

Current status and challenges of biohydrogels for applications as supercapacitors and secondary batteries

Progress in the chemical sciences has formed the world we live in, both on a macroscopic and on a nanoscopic scale. The last decade has witnessed the development of high performance materials that store charge in many ways: from solar cells to fuel cells, and from batteries to supercapacitor devices. One could argue that inorganic hybrid materials have played a central, starring role for the assembly of various electrochemical energy conversion systems. However, energy conversion systems fabricated from biopolymers has just emerged as a new prospect. Here, we summarize the main research results on the attractive use of biohydrogels for the fabrication of either conductive electrolytes or electrodes for battery science and technology.

Hybrid nanofibers from biodegradable polylactide and polythiophene for scaffolds

Hybrid scaffolds constituted of polylactide (PLA) as a biodegradable polymer and poly(3-thiophene methyl acetate) (P3TMA) as an electroactive polymer were prepared and studied. Both polymers had a similar solubility and consequently could be easily electrospun using a common solvent. Electrospinning operational parameters were optimized to get continuous micro/nanofibers with a homogeneous diameter that ranged between 600 and 900 nm depending on the PLA–P3TMA ratio. Electrospinning was only effective when the P3TMA content was at maximum 50 wt%. The incorporation of P3TMA slightly decreased the fibre diameter, led to smoother fibre surfaces and gave rise to some heterogeneous clusters inside the fibers. PLA was highly oriented inside the electrospun fibers and able to easily cold crystallize by heating. Thermal degradation was not highly influenced by the presence of P3TMA, although the onset temperature slightly increased since the first decomposition step of PLA was prevented. New scaffolds had promising electrochemical properties and even provided a good substrate for cell adhesion and cell proliferation. Therefore, these hybrid materials are suitable to improve the cellular response towards physiological processes.