Iline Steyaert

Polycaprolactone and polycaprolactone/chitosan nanofibres functionalised with the pH-sensitive dye Nitrazine Yellow

Nanofibres functionalised with pH-sensitive dyes could greatly contribute to the development of stimuli-responsive materials. However, the application of biocompatible polymers is vital to allow for their use in (bio)medical applications. Therefore, this paper focuses on the development and characterisation of pH-sensitive polycaprolactone (PCL) nanofibrous structures and PCL/chitosan nanofibrous blends with 20% chitosan. Electrospinning with added Nitrazine Yellow molecules proved to be an excellent method resulting in pH-responsive non-wovens. Unlike the slow and broad response of PCL nanofibres (time lag of more than 3h), the use of blends with chitosan led to an increased sensitivity and significantly reduced response time (time lag of 5 min). These important effects are attributed to the increased hydrophilic nature of the nanofibres containing chitosan. Computational calculations indicated stronger interactions, mainly based on electrostatic interactions, of the dye with chitosan (ΔG of -132.3 kJ/mol) compared to the long-range interactions with PCL (ΔG of -35.6 kJ/mol), thus underpinning our experimental observations. In conclusion, because of the unique characteristics of chitosan, the use of PCL/chitosan blends in pH-sensitive biocompatible nanofibrous sensors is crucial.

Dye immobilization in halochromic nanofibers through blend electrospinning of a dye-containing copolymer and polyamide-6

‘Smart’ materials can be defined as materials that respond to a certain stimulus with a change in their properties. A specific class herein is halochromic textiles, i.e. fibrous materials that change color with pH. Such halochromic textiles play an important role in the continuous monitoring and visual reporting of the pH with applications in various fields, such as wound treatment and protective clothing. pH-sensitive nanofibrous nonwovens have high sensitivity and a fast response time, and are mostly fabricated by introducing a pH-responsive dye via dye-doping of the feed mixture before fabrication. However, this method suffers from leaching of the dye, which is an undesirable effect that not only reduces the output signal strength but can also be detrimental to the environment by causing, for instance, toxicological responses. In this paper, a new strategy is demonstrated for the reduction of dye leaching in electrospun, nanofibrous materials. Through blend electrospinning of polyamide-6 (PA6) with a dye-functionalized copolymer, large sheets of uniform, halochromic nanofibrous material can be fabricated showing a fast pH-sensitive color change. Polymeric entanglements within the nanofiber are proposed to immobilize the dye-functionalized copolymer in the PA6 matrix, resulting in drastically reduced dye leaching. Such stable nanofibrous, PA6-based, halochromic materials are particularly interesting in the design of new colorimetric sensors applicable in several sectors, including the biomedical field, agriculture, safety and technical textiles.

Blend electrospinning of dye-functionalized chitosan and poly(ε-caprolactone): towards biocompatible pH-sensors

Fast-response and easy-to-visualize colorimetric nanofibrous sensors show great potential for visual and continuous control of external stimuli. This makes them applicable in many fields, including wound management, where nanofibers serve as an optimal support material. In this paper, fast responding and user-friendly biocompatible, halochromic nanofibrous sensors are successfully fabricated by incorporating the halochromic dyes Methyl Red and Rose Bengal inside a chitosan/poly(ε-caprolactone) nanofibrous matrix. The commonly applied dye-doping technique frequently suffers from dye-leaching, which not only reduces the sensors sensitivity over time but can also induce adverse effects. Therefore, in this work, dye-immobilization is accomplished by covalent dye-modification of chitosan before blend electrospinning. It is shown that efficient dye-immobilization with minimal dye-leaching is achieved within the biomedical relevant pH-region, without significantly affecting the halochromic behavior of the dyes. This is in contrast to the commonly applied dye-doping technique and other dye-immobilization strategies stated in literature. Moreover, the nanofibers show high and reproducible pH-sensitivity by providing an instantaneous color change in response to change in pH in aqueous medium and when exposed to acidic or basic gases. The results stated within this work are of particular interest for natural (bio)polymers for which covalent modification combined with electrospinning provides a universal method for versatile dye-functionalization of large area nanofibrous membranes with proper dye-immobilization.

Nanofibre-Based Sensors for Visual and Optical Monitoring

Sensors supplying a change in optical properties, easily detectable with the naked eye (visual) or inexpensive equipment such as compact spectrometers (optical), are a very powerful tool to visualise a wide range of parameters, including temperature, light, pH and concentration of chemical substances. Most of these sensors rely on indicator compounds showing a change in optical absorbance (colour) or fluorescence under the influence of a certain parameter. Halochromic dyes, for instance, change colour with pH. Since the use of nanofibres improves sensor sensitivity and response time due to their large surface area to volume ratio, the incorporation of indicator compounds into nanofibres is one of the current challenges in sensor design. This chapter discusses the production of colorimetric and fluorescent nanofibrous membranes for visual and optical monitoring (Sects. 7.3 and 7.4), supplemented by some fundamental information on those sensing systems (Sect. 7.2) and some interesting applications (Sect. 7.5).

Nanostructured Hydrogels by Blend Electrospinning of Polycaprolactone/Gelatin Nanofibers

Nanofibrous membranes based on polycaprolactone (PCL) have a large potential for use in biomedical applications but are limited by the hydrophobicity of PCL. Blend electrospinning of PCL with other biomedical suited materials, such as gelatin (Gt) allows for the design of better and new materials. This study investigates the possibility of blend electrospinning PCL/Gt nanofibrous membranes which can be used to design a range of novel materials better suited for biomedical applications. The electrospinnability and stability of PCL/Gt blend nanofibers from a non-toxic acid solvent system are investigated. The solvent system developed in this work allows good electrospinnable emulsions for the whole PCL/Gt composition range. Uniform bead-free nanofibers can easily be produced, and the resulting fiber diameter can be tuned by altering the total polymer concentration. Addition of small amounts of water stabilizes the electrospinning emulsions, allowing the electrospinning of large and homogeneous nanofibrous structures over a prolonged period. The resulting blend nanofibrous membranes are analyzed for their composition, morphology, and homogeneity. Cold-gelling experiments on these novel membranes show the possibility of obtaining water-stable PCL/Gt nanofibrous membranes, as well as nanostructured hydrogels reinforced with nanofibers. Both material classes provide a high potential for designing new material applications.