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Advanced Drug Delivery Reviews (v.59, #14)
Intersection of nanoscience and modern surface analytical methodology
by Mark Gumbleton (Theme Editor) (pp. 1383-1383).
Formation of fibers by electrospinning by Gregory C. Rutledge; Sergey V. Fridrikh (pp. 1384-1391).
Electrostatic fiber formation, also known as “electrospinning”, has emerged in recent years as the popular choice for producing continuous threads, fiber arrays and nonwoven fabrics with fiber diameters below 1 μm for a wide range of materials, from biopolymers to ceramics. It benefits from ease of implementation and generality of use. Here, we review some of the basic aspects of the electrospinning process, as it is widely practiced in academic laboratories. For purposes of organization, the process is decomposed into five operational components: fluid charging, formation of the cone-jet, thinning of the steady jet, onset and growth of jet instabilities that give rise to diameter reduction into the submicron regime, and collection of the fibers into useful forms. Dependence of the jetting phenomenon on operating variables is discussed. Continuum level models of the jet thinning and jet instability are also summarized and put in some context.
Keywords: Electrospinning; Nanofiber; Electrohydrodynamics
Functional electrospun nanofibrous scaffolds for biomedical applications by Dehai Liang; Benjamin S. Hsiao; Benjamin Chu (pp. 1392-1412).
Functional nanofibrous scaffolds produced by electrospinning have great potential in many biomedical applications, such as tissue engineering, wound dressing, enzyme immobilization and drug (gene) delivery. For a specific successful application, the chemical, physical and biological properties of electrospun scaffolds should be adjusted to match the environment by using a combination of multi-component compositions and fabrication techniques where electrospinning has often become a pivotal tool. The property of the nanofibrous scaffold can be further improved with innovative development in electrospinning processes, such as two-component electrospinning and in-situ mixing electrospinning. Post modifications of electrospun membranes also provide effective means to render the electrospun scaffolds with controlled anisotropy and porosity. In this article, we review the materials, techniques and post modification methods to functionalize electrospun nanofibrous scaffolds suitable for biomedical applications.
Keywords: Electrospinning; Nanofiber; Scaffold; Biomedical applications; Copolymers; Mixtures; Modifications
Nanofiber technology: Designing the next generation of tissue engineering scaffolds by Catherine P. Barnes; Scott A. Sell; Eugene D. Boland; David G. Simpson; Gary L. Bowlin (pp. 1413-1433).
Tissue engineering is an interdisciplinary field that has attempted to utilize a variety of processing methods with synthetic and natural polymers to fabricate scaffolds for the regeneration of tissues and organs. The study of structure–function relationships in both normal and pathological tissues has been coupled with the development of biologically active substitutes or engineered materials. The fibrillar collagens, types I, II, and III, are the most abundant natural polymers in the body and are found throughout the interstitial spaces where they function to impart overall structural integrity and strength to tissues. The collagen structures, referred to as extracellular matrix (ECM), provide the cells with the appropriate biological environment for embryologic development, organogenesis, cell growth, and wound repair. In the native tissues, the structural ECM proteins range in diameter from 50 to 500 nm. In order to create scaffolds or ECM analogues, which are truly biomimicking at this scale, one must employ nanotechnology. Recent advances in nanotechnology have led to a variety of approaches for the development of engineered ECM analogues. To date, three processing techniques (self-assembly, phase separation, and electrospinning) have evolved to allow the fabrication of nanofibrous scaffolds. With these advances, the long-awaited and much anticipated construction of a truly “biomimicking” or “ideal” tissue engineered environment, or scaffold, for a variety of tissues is now highly feasible. This review will discuss the three primary technologies (with a focus on electrospinning) available to create tissue engineering scaffolds that are capable of mimicking native tissue, as well as explore the wide array of materials investigated for use in scaffolds.
Keywords: Nanofibers; Electrospinning; Tissue engineering; Scaffold
Pharmaceutical applications of confocal laser scanning microscopy: The physical characterisation of pharmaceutical systems by Samuel R. Pygall; Joanne Whetstone; Peter Timmins; Colin D. Melia (pp. 1434-1452).
The application of confocal laser scanning microscopy (CLSM) to the physicochemical characterisation of pharmaceutical systems is not as widespread as its application within the field of cell biology. However, methods have been developed to exploit the imaging capabilities of CLSM to study a wide range of pharmaceutical systems, including phase-separated polymers, colloidal systems, microspheres, pellets, tablets, film coatings, hydrophilic matrices, and chromatographic stationary phases. Additionally, methods to measure diffusion in gels, bioadhesives, and for monitoring microenvironmental pH change within dosage forms have been utilised. CLSM has also been used in the study of the physical interaction of dosage forms with biological barriers such as the eye, skin and intestinal epithelia, and in particular, to determine the effectiveness of a plethora of pharmaceutical systems to deliver drugs through these barriers. In the future, there is continuing scope for wider exploitation of existing techniques, and continuing advancements in instrumentation.
Keywords: Abbreviations; CLSM; Confocal Laser Scanning Microscopy; MCC; Microcrystalline cellulose; FRAP; Fluorescence recovery after photobleaching; HPMC; Hydroxypropylmethylcellulose; NMR; Nuclear Magnetic Resonance; PLA-PEG; Poly(lactic acid)-poly(ethylene glycol); FITC; Fluorescein isothiocyanate; PLGA; Poly(lactic-glycolic) acidConfocal laser scanning microscopy; Fluorescence; Physicochemical; Characterisation; Pharmaceuticals; Dosage form; Delivery systems; Chromatography; Profilometry
Scanning probe microscopy in the field of drug delivery by Ya Tsz A. Turner; Clive J. Roberts; Martyn C. Davies (pp. 1453-1473).
The scanning probe microscopes (SPMs) are a group of powerful surface sensitive instruments which when used complimentarily with traditional analytical techniques can provide invaluable, definitive information aiding our understanding and development of drug delivery systems. In this review, the main use of the SPMs (particularly the atomic force microscopy (AFM)) and their successes in forwarding drug delivery are highlighted and categorised into two interlinked sections namely, preformulation and formulation. SPM in preformulation concentrates on applications in pharmaceutical processes including, crystal morphology and modification, discriminating polymorphs, drug dissolution and release, solid state stability and interaction. The ability of the AFM to detect forces between different surfaces and at the same time to operate in liquids or controlled humidity and defined temperatures has also been particularly useful in the study of drug delivery. In formulation, the use of SPMs in different drug delivery systems is discussed in light of different host entry routes.
Keywords: Drug delivery; SPM; AFM; Drug formulations; Surface characterisation
The importance of surface energetics of powders for drug delivery and the establishment of inverse gas chromatography by Graham Buckton; Hardyal Gill (pp. 1474-1479).
Powders are complex systems with more than one value for surface energy. The presence of different faces, defects, physical forms and impurities will alter the surface properties. There are few good ways to measure powder surface energies, with vapour sorption, especially inverse gas chromatography (IGC) being a logical choice. The significance of surface energy is reviewed briefly, as is the difference between contact angle and IGC data. The utility of IGC for studies of batch to batch variability and some issues relating to finding a suitable number to describe a complex range of surface energies are discussed. The utility of IGC in studies of the amorphous state is shown, where there is value in being able to monitor molecular mobility thresholds, glass transition, collapse and crystallisation, as well as relaxation and its impact on surface energy. The conclusion is that the complexity of powders means that scientists should not expect simple correlations between measurements and performance, but that correlations are likely to be there if the correct data are recorded in the most appropriate way.
Keywords: Inverse gas chromatography; Surface energy; Contact angle; Glass transition; Amorphous forms; Batch variability