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Nanomaterials are highly popular within tissue engineering, which has had more fusion with nanotechnology over recent years, with regenerative medicine aims playing a prominent role. Anodic aluminum oxide (AAO) is a nanoporous structure likened to a honeycomb with high-density arrays of uniform and parallel pores.
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With the unique properties of nanomaterials, such as having a high surface-to-volume ratio and a diverse morphology and structure, they have become a desirable material for different applications.
Nanomaterials are sometimes challenging to fabricate, but anodic aluminum oxides' easy engineering process ensures its applicability for various uses. This includes mammalian cell culture, biofunctionalization, drug delivery, or even a nanosensor.
Aluminum is one of the most abundant metals. The refinement and manufacturing of alumina from bauxite ore led by Sir Humphrey Davy, physicist Hans Christian Ersted, and Karl Bayer has meant that it has become the most highly produced non-ferrous metal for various applications.
Anodic aluminum oxide (AAO), specifically, has acquired attention due to its potential for use as either a scaffold or as a mold to create polymeric scaffolds, mainly for its unique mechanical and chemical properties. It can also be used for advancements in optics.
AAO has illustrated a high level of controllability with other benefits such as cost efficiency, no toxicity, tunable pore density, and a highly ordered structure. Due to its formation, AAO is a desirable nanostructure for use as a scaffold which would require good cell adhesion and ensure proliferation. The high surface-to-volume ratio of this nanomaterial also supports natural biological behavior and roles of cells. The controllable pore size of anodic aluminum oxide further aids the suitability of AAO within native tissue.
The role of scaffolds is of great importance in tissue engineering with the regeneration of solid components within the body such as the skin, bone, ligaments, cartilage, and more. These scaffolds can be used to replace lost tissue either temporarily or permanently, with the aim being to create a suitable cultivatable environment that will guide the proliferation of cells.
However, this consists of matching the scaffold to the native tissue and ensuring similar chemical and mechanical properties between them.
With AAO carrying this ability through manipulation of pore size and density within the same scale as naturally-found cell dimensions, such as in a range of 108–1010 pores cm−2, this obstacle can be overcome.
Anodic aluminum oxide has shown great potential as a nanomaterial due to its manipulable and adaptable state; its physical and mechanical properties can be personalized and altered to meet various applications.
This can include undergoing the process of anodization. First developed in the mid-1920s, anodization is an electrochemical process that can be used to increase the thickness of the oxide layer that can be found on the surface of metal components. It can essentially convert the metal surface, such as aluminum, into a more durable and corrosion-resistant state with an anodic oxide finish.
The two main methods can consist of mild and hard anodization, with mild anodization producing self-ordered and straight pores, and hard anodization being more disordered and non-uniform with its creation of pores, leading to lower pore density.
While mild anodization is more uniform, it is also a slower process which causes this method to be less useful; however, hard anodization is faster but provides a more disordered result.
The obstacles of these two classical anodization methods have led to the development of two other methods, such as cyclic and pulse anodization, which consist of a mix of the two conventional processes resulting in highly ordered nanopores.
The biomedical applications of anodic aluminum oxide have been promising due to its highly ordered structure. With recent advancements in nanotechnology, fabricating this nanomaterial is cost-efficient and more accessible.
The use of alumina, which is formed from an electrochemical process in an acidic solution that produces aluminum oxide, also has significant applications in this field of science. Applications include shaping 3D complex porous nanostructures to indirect uses as a mold for the production of nanowires, nanofibers, or nanotubes.
Polymeric materials are the most common material that alumina membrane molds can produce.
Additionally, AAO can aid in the creation of biodegradable poly (3-caprolactone) (PCL) nanowires that can be used for neural cell culture, where PCL can be put on top of the aluminum oxide membrane for the penetration of PCL into the nanochannel. This combination has provided increased cell adhesion as well as propagation.
Other experiments and applications have also included PCL nanowires with AAO being utilized for a scaffold to advance bone tissue engineering. With bone engineering being one of the most difficult specialisms of regenerative medicine, experiments by Porter et al. (2009) have illustrated that PCL nanowire surfaces for rat bone marrow stem cells increased adhesion, proliferation, alkaline phosphate levels, and mineralization. Other researchers have also found AAO molds as a useful method to fabricate hydroxyapatite nanowires, which had similarity to that being found in native bone.
The applications of AAO can be seen as being revolutionary for the field of regeneration and biomedical sciences, with prominent applications paving the way for more advanced treatments that could improve patient care quality.
The use of AAO for optical sensors is also a significant application that can innovate biomedical research, with AAO based optical biosensors having features such as high selectivity, specificity, and reusable. The most common optical methods which utilize AAO include Surface-Enhanced Raman Scattering (SERS), Surface Plasmon Resonance (SPR), reflectometric Interference Spectroscopy (RIfS), and Photoluminescence Spectroscopy (PL). These advanced optical biosensors have great advantages, such as being fast and efficient and an incomparable detection limit that is lower than conventional laboratory-based equipment.
With further research aiming to illuminate the ambiguities and limitations of AAO in vitro and in vivo, such as through trials and its advancement in other areas such as optics, its potential as a revolutionary nanomaterial is promising. This can further innovate nanotechnology and industries from electronic to medical, diagnostic to therapeutic in a very tangible way.
Continue reading: Industrial Applications of Nanostructured Carbon.
Davoodi, E., Zhianmanesh, M., Montazerian, H. et al. (2020) Nano-porous anodic alumina: fundamentals and applications in tissue engineering. J Mater Sci: Mater Med 31, 60 Available at: https://doi.org/10.1007/s10856-020-06398-2
Feng, S. and Ji, W., (2021) Advanced Nanoporous Anodic Alumina-Based Optical Sensors for Biomedical Applications. Frontiers in Nanotechnology, 3. Available at: https://doi.org/10.3389/fnano.2021.678275
Porter, J., Henson, A. and Popat, K., (2009) Biodegradable poly(ɛ-caprolactone) nanowires for bone tissue engineering applications. Biomaterials, 30(5), pp.780-788. Available at: https://doi.org/10.1016/j.biomaterials.2008.10.022
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Marzia Khan is a lover of scientific research and innovation. She immerses herself in literature and novel therapeutics which she does through her position on the Royal Free Ethical Review Board. Marzia has a MSc in Nanotechnology and Regenerative Medicine as well as a BSc in Biomedical Sciences. She is currently working in the NHS and is engaging in a scientific innovation program.
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