Nanoporous anodic aluminium oxide: Advances in surface engineering and emerging applications

Anodic aluminium oxide (AAO) has been investigated and utilized in numerous products for almost a century. But the rapidly increasing interest in nanoscale materials and their outstanding properties has propelled nanoporous AAO to the fore as one of the most popular nanomaterial with applications across a gamut of areas including molecular separation, catalysis, energy generation and storage, electronics and photonics, sensors and biosensors, drug delivery and template synthesis. Material fabrication of AAO is based on facile and inexpensive electrochemical anodization with the self-ordering process of nanopores not requiring any lithography or templating, and the outcome of the process are perfectly ordered and size controlled nanopores with distinctive pore geometries. Recent research on AAO is characterized by a remarkable trajectory of innovation, in particular with regards to control of surface functionality and, concomitantly, to the design of intricate structural features such as modulated, branched, and multilayered pore architectures. This review illuminates research on recent development of AAO focussing on surface and structural engineering, and on emerging applications. Key examples and critical preparative issues and resulting improvements sparking opportunities for further applications in AAO properties are discussed. We conclude this review with an outlook providing a critical perspective on future trends on surface and structural engineering of AAO.

Ag2SO4 decorated with fluorescent Agn nanoclusters

Here we report on the production of an Ag2SO4/Ag2O mixed-grain powder during the anodization of Ag foil in a HF-H2SO4 electrolyte. We propose that there are three competing reactions during the anodization process: (i) the production of Ag2O at the Ag foil anode surface from the presence of water in the electrolyte, (ii) the dissolution of the Ag2O in the presence of HF releasing Ag+ ions, (iii) the precipitation of Ag+ and SO4 2− ions, as Ag2SO4 on the Ag foil anode surface. This co-precipitation/dissolution process ultimately results in a mixed-grain powder. We then show that the Ag2O embedded within the mixedgrain is photo-decomposed to produce highly fluorescent silver nanoclusters (Agn) which decorate the Ag2SO4 crystals. The Ag2SO4 salt offers a stable matrix for the photo-decomposed Agn nanoclusters to emit their strong fluorescence.

Applications of modern sensors and wireless technology in effective wound management

The management of chronic wounds has emerged as a major health care challenge during the 21st century consuming, significant portions of health care budgets. Chronic wounds such as diabetic foot ulcers, leg ulcers, and pressure sores have a significant negative impact on the quality of life of affected individuals. Covering wounds with suitable dressings facilitates the healing process and is common practice in wound management plans. However, standard dressings do not provide insights into the status of the wound underneath. Parameters such as moisture, pressure, temperature and pH inside the dressings are indicative of the healing rate, infection, and wound healing phase. But owing to the lack of information available from within the dressings, these are often changed to inspect the wound, disturbing the normal healing process of wounds in addition to causing pain to the patient. Sensors embedded in the dressing would provide clinicians and nurses with important information that would aid in wound care decision making, improve patient comfort, and reduce the frequency of dressing changes. The potential benefits of this enabling technology would be seen in terms of a reduction in hospitalization time and health care cost. Modern sensing technology along with wireless radio frequency communication technology is poised to make significant advances in wound management. This review discusses issues related to the design and implementation of sensor technology and telemetry systems both incorporated in wound dressings to devise an automated wound monitoring technology, and also surveys the literature available on current sensor and wireless telemetry systems.

MALDI MS imaging analysis of apolipoprotein E and lysyl oxidase-like 1 in human lens capsules affected by pseudoexfoliation syndrome

Pseudoexfoliation (PEX) syndrome is an age-related systemic disease of the extracellular matrix, characterized by the presence of amyloid-like fibrillar deposits on the anterior lens capsule. The pathological deposits (PEX material) can obstruct aqueous outflow leading to increased intraocular pressure that in turn can result in glaucoma. PEX syndrome is the most common risk factor for glaucoma. In our previous work, we reported a protocol for the analysis of human lens capsules by MALDI MS imaging. Here, we extend our previous work applying the developed protocol to the analysis of human lens capsules affected by PEX syndrome. We focus our investigation on known components of the PEX material, namely lysyl oxidase-like 1 (LOXL1) and apolipoprotein E (APOE). Our results show that LOXL1 is more abundant in the deposits in the iris region and, alternatively APOE is concentrated in the PEX material accumulated in the pupillary area of the anterior lens capsule. Furthermore, we identify potentially relevant post-translational modifications which may have an important role in promoting the cross-linking processes in PEX syndrome and stabilize aggregate structures within the proteinaceous PEX material.

Surface engineering for long-term culturing of mesenchymal stem cell microarrays

The cell microarray format can recreate a multitude of cell microenvironments on a single chip using only minimal amounts of reagent. In this study, we describe surface modifications to passivate cell microarrays, aiming to adapt the platform to the study of stem cell behavior over long-term culture periods. Functionalization of glass slides with (3-glycidyloxypropyl) trimethoxysilane enabled covalent anchoring of extracellular matrix proteins on microscale spots printed by a robotic contact printer. Subsequently, the surface was passivated by bovine serum albumin (BSA) or poly(ethylene glycol)bisamine (A-PEG) with molecular weights of 3000, 6000, and 10 000 Da. Cloud-point conditions for A-PEG grafting were attained that were compatible with protein deposition. Passivation strategies were assessed by culturing mesenchymal stem cells on the microarray platform. While both BSA and A-PEG passivation initially blocked cell adhesion between the printed spots, only A-PEG grafting was able to maintain cell pattern integrity over the entire culture period of 3 weeks.

Exploring the mesenchymal stem cell niche using high throughput screening

In the field of stem cell technology, future advancements rely on the effective isolation, scale-up and maintenance of specific stem cell populations and robust procedures for their directed differentiation. The stem cell microenvironment – or niche – encompasses signal inputs from stem cells, supporting cells and from the extracellular matrix. In this context, the contribution of physicochemical surface variables is being increasingly recognised. This paradigm can be exploited to exert control over cellular behaviour. However, the number of parameters at play, and their complex interactions, presents a formidable challenge in delineating how the decisions of cell fate are orchestrated within the niche. Additionally, in the case of mesenchymal stem cells (MSC), more than one type of stem cell niche has been identified. By employing high throughput screening (HTS) strategies, common and specific attributes of each MSC niche can be probed. Here, we explore biological, chemical and physical parameters that are known to influence MSC self-renewal and differentiation. We then review techniques and strategies that allow the HTS of surface properties for conditions that direct stem cell fate, using MSC as a case study. Finally, challenges in recapturing the niche, particularly its three dimensional nature, in surface-based HTS formats are discussed.

High-throughput characterisation of osteogenic differentiation of human mesenchymal stem cells using pore size gradients on porous alumina

The ability to control the cellular response is of critical importance when designing advanced biomaterials for applications in tissue engineering and regenerative medicine. An important aspect of biointerfacial interactions is surface topography at the nanoscale and therefore this needs to be taken into consideration. Here, a pore size gradient in porous alumina (pAl) was fabricated with pore sizes ranging from 50 nm to 3 μm. The attachment behaviour and osteogenesis of human mesenchymal stem cells (hMSCs) was investigated along this topography gradient for up to 2 weeks. Generally, cell attachment density and spreading area decreased with increasing pore size. Pore wall width and solid surface fraction also played a key role in cell adhesion. After 2 weeks, osteogenesis of hMSCs was enhanced by porous topography with a pore size of 120–230 nm in diameter and 10 nm pore wall width, compared with other topographies of the gradient. The results demonstrate that the gradient format allows in-depth high-throughput screening of surface parameters that are important for the control of mammalian cell behaviour, thereby advancing the development of new and improved biomaterials for e.g. orthopaedic and tissue engineering applications.

Biosensors and Point-of-Care Devices for Bacterial Detection: Rapid Diagnostics Informing Antibiotic Therapy

With an exponential rise in antimicrobial resistance and stagnant antibiotic development pipeline, there is, more than ever, a crucial need to optimize current infection therapy approaches. One of the most important stages in this process requires rapid and effective identification of pathogenic bacteria responsible for diseases. Current gold standard techniques of bacterial detection include culture methods, polymerase chain reactions, and immunoassays. However, their use is fraught with downsides with high turnaround time and low accuracy being the most prominent. This imposes great limitations on their eventual application as point-of-care devices. Over time, innovative detection techniques have been proposed and developed to curb these drawbacks. In this review, a systematic summary of a range of biosensing platforms is provided with a strong focus on technologies conferring high detection sensitivity and specificity. A thorough analysis is performed and the benefits and drawbacks of each type of biosensor are highlighted, the factors influencing their potential as point-of-care devices are discussed, and the authors' insights for their translation from proof-of-concept systems into commercial medical devices are provided

Spatially Controlled Surface Modification of Porous Silicon for Sustained Drug Delivery Applications

Abstract A new and facile approach to selectively functionalize the internal and external surfaces of porous silicon (pSi) for drug delivery applications is reported. To provide a surface that is suitable for sustained drug release of the hydrophobic cancer chemotherapy drug camptothecin (CPT), the internal surfaces of pSi films were first modified with 1-dodecene. To further modify the external surface of the pSi samples, an interlayer was applied by silanization with (3-aminopropyl)triethoxysilane (APTES) following air plasma treatment. In addition, copolymers of N-(2-hydroxypropyl) acrylamide (HPAm) and N-benzophenone acrylamide (BPAm) were grafted onto the external pSi surfaces by spin-coating and UV crosslinking. Each modification step was verified using attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, water contact angle (WCA) measurements, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). In order to confirm that the air plasma treatment and silanization step only occurred on the top surface of pSi samples, confocal microscopy was employed after fluorescein isothiocyanate (FITC) conjugation. Drug release studies carried out over 17 h in PBS demonstrated that the modified pSi reservoirs released CPT continuously, while showing excellent stability. Furthermore, protein adsorption and cell attachment studies demonstrated the ability of the graft polymer layer to reduce both significantly. In combination with the biocompatible pSi substrate material, the facile modification strategy described in this study provides access to new multifunctional drug delivery systems (DDS) for applications in cancer therapy

Interaction of antibiotics with lipid vesicles on thin film porous silicon using reflectance interferometric fourier transform spectroscopy

The ability to observe interactions of drugs with cell membranes is an important area in pharmaceutical research. However, these processes are often difficult to understand due to the dynamic nature of cell membranes. Therefore, artificial systems composed of lipids have been used to study membrane properties and their interaction with drugs. Here, lipid vesicle adsorption, rupture, and formation of planar lipid bilayers induced by various antibiotics (surfactin, azithromycin, gramicidin, melittin and ciprofloxacin) and the detergent dodecyl-b-d-thiomaltoside (DOTM) was studied using reflective interferometric Fourier transform spectroscopy (RIFTS) on an oxidized porous silicon (pSi) surface as a transducer. The pSi transducer surfaces are prepared as thin films of 3 μm thickness with pore dimensions of a few nanometers in diameter by electrochemical etching of crystalline silicon followed by passivation with a thermal oxide layer. Furthermore, the sensitivity of RIFTS was investigated using three different concentrations of surfactin. Complementary techniques including atomic force microscopy, fluorescence recovery after photobleaching, and fluorescence microscopy were used to validate the RIFTS-based method and confirm adsorption and consequent rupture of vesicles to form a phospholipid bilayer upon the addition of antibiotics. The method provides a sensitive and real-time approach to monitor the antibiotic-induced transition of lipid vesicles to phospholipid bilayers.