Synthetic stimuli-responsive ‘smart’ fibers
Graphical abstract
Introduction
The choice of materials for fibers hinges on the relevant biomedical application. For example, the fiber must incorporate biocompatibility with other features such as mechanical strength, hydrophilicity and have dynamic properties such as a change in conformation initiated by either a change in temperature, pH or light [1••, 2]. These dynamic properties should ideally be switchable [1••, 2]. In particular, polymers have emerged as a strong candidate in biomedical applications and are not just limited to natural polymers but also a wide range of synthetic polymers. The most recent developments have been around the creation of smart materials that use synthetic polymers. For example, synthetic polymers have been used to investigate cell orientations that change in response to manipulation of microfiber diameter [2].
A smart material has the ability to respond to stimuli such as pH, magnetic field or light and it is this dynamic property that distinguishes these ‘stimuli-responsive materials’ as a new class of material that has undergone extensive research over the last two decades [3••]. For example, controlled drug release has been achieved through electrical stimulus of magnetic/temperature responsive materials for biomedical applications [4, 5]. Smart materials can also be precursors for the fabrication of ‘smart fibers’, a type of fiber that can respond to stimulus and one that can be fabricated by conventional methods such as electrospinning [6]. An exciting property of these materials is that they have very fast response rates due to their inherent design features. For example, Figure 1 illustrates the ‘on-off’ drug release from temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm) electrospun fibers. Control of the PNIPAAm fiber diameter from the micro-scale to nano-scale, allows a stimuli response that can be tuned. Additionally, these fibers can be used as fast on-off switchable materials making them a dynamic material that can find many biomaterial applications [6].
The following sections will review some of the varieties of smart fibers and their key fabrication methods as well as some of their applications.
Section snippets
Self-assembly
Lipid membranes, which control cellular processes, assemble from a hydrophobic tail and a hydrophilic head group [7]. This natural organisation of life is driven by non-covalent forces and it is known that polymer fibers and liquid crystals (LLC) all self-assemble in solution based on the same principles that drive natural molecular assembly [8]. Recently, Saito et al. [9] have developed a stimuli-responsive self-assembled system from lyotropic LLCs composed of cyclic ethynylhelicene oligomers.
Temperature-responsive fibers
When a polymer changes its phase in response to a change in temperature the material is under dynamic control and this material property is being widely utilized in the biotechnology field. The internal structure and wettability of temperature response fibers has made them ideal candidates for applications in drug delivery, tissue engineering and biomolecule separation assays [23, 24]. PNIPAAm has a sharp, reversible change in its phase at 32 °C due to the dehydration of the polymer which drives
Sensors
Light, electrochemical signal, mass, magnetism and thermal effects can all be measured by sensors, and the number and variety of sensors that exist today is astounding [39, 40]. Smart polymers are of course being used as sensors and the combination of smart materials with existing technologies can lead to new sensors and improved sensitivity [39].
PNIPAAm electrospun fibers have been investigated for their utility as temperature sensors [41]. N-methylolacrylamide (NMA) and N-isopropyl acrylamide
Conclusion and future trends
In this review we have given a brief overview of the variety of methods used to synthesize smart fibers. It is indeed possible to control the morphology of fibers be it through non covalent forces during self-assembly fabrication or by control over a fluidic system using electrospinning, writing or microfluidic devices. Fabricating fibers which are dynamic and responsive is crucial for integration into a biological system. The many different precursor materials used make it possible to
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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