Nanofibres to nanocomposites - Electrospinning trends for biomedical use

Materials World magazine
,
8 Apr 2013

Electrospinning of nanofibres is generating interest across a wide range of industry sectors, in particular for biomedical applications. Professor Bob Stevens of Nottingham Trent University, UK, and Dr Barry Park and Dr Felicity Sartain from the UK’s NanoKTN, take a look at growing trends in the UK nanofibres market.

Nano in a nutshell
A nanofibre can be defined as having a diameter of less than one micron and an aspect ratio of greater than 50. However, a nanomaterial has been defined as having at least one dimension less than 100nm, so on this basis, the diameter should be less than 100nm. As such, the scope of nanofibres may be reported in different ways on different occasions.

While the term nano has only recently crept into everyday use, its roots stem back to the Second World War. It was in the 1930s in the former Soviet Union that Russian physicist Igor Petryanov first used an electric field to produce filter materials, with the first commercial use mainly to protect against nuclear- active aerosols. Electrospinning has been used ever since to add value to filtration products, however, it wasn’t until the late 1990s that research activity in the field surged. Currently, a number of elecrospun materials are being developed for a wide range of markets, including the aerospace, automotive and healthcare sectors.

Electrospinning is a nanofibre synthesis process that is most frequently used to transform a homogenous liquid mixture consisting of polymers of varying molecular weights, mixed with a cocktail of solvents and a range of additives into non-woven and/or aligned nanofibre fabrics. The process spins fibres of diameters ranging from 10 to several thousand nanometers, producing electrospun fibre mats that can be nanofibrous, microfibrous or a mixture of both.

A growing market
Polymeric nanofibre fabrics offer a number of benefits, including high surface area to volume ratio, flexibility and controlled porosity. These materials can be made from a wide range of polymers and polymer blends, which, when incorporated with a variety of nanoparticle materials and molecular additives, can confer varying hydrophobic or hydrophilic nature on the final material. With such a diverse range of properties, it comes as no surprise that a 2012 Business Communication Company Research (BCC Research) market report forecasted growth of the global nanofibres market from US$183m in 2012 to US$835m in 2017. While other market reports agree with these figures, a 2010 survey by Future Markets Inc suggested a slower growth rate, to just US$334m in 2017. Nonetheless, this remains a market expected to grow in excess of 30% a year. The mechanical/chemical sector is estimated to account for almost 75% of this market value in the next few years, with the electronics market forecast to be the next largest sector.

Currently, the leading players in the nanofibres market include DuPont, Hollingsworth and Vose, Kuraray and Teijen, although up-and-coming innovative companies such as Elmarco, Finetex and Nanoval are hot on their heels. In the UK, the realisation of a controlled electrospinning process environment for consistent production of nanofibre materials has been realised, providing confidence for investors looking to engage with new start-up companies focusing on exploiting the properties of nanofibre-enabled products – predominantly for the healthcare and life sciences sector.

One example is The Electrospinning Company in Oxford, UK, whose portfolio of highly consistent electrospun nanofibre scaffolds is enabling the life sciences sector to grow cells in 3D. Currently, the company is developing a 96 well plate for 3D cellular assays for drug discovery, profiling and toxicity tests, in a project supported by the Technology Strategy Board. It is also participating in an EU-funded project called ReLiver, with the aim of developing scaffolds for functioning liver cells that could be used in invitro assays and transplantable organoids. Another UK SME, Cheshire-based SpheriTech, has launched a range of novel 3D scaffolds using materials that consist entirely of naturally occurring amino acids and fatty acids.

A 2007 spin-out from Smith & Nephew in 2007, York-based Neotherix is focusing on soft tissue repair, in particular, skin and oral cavity wounds as well as post-surgical applications. Its core technology is electrospun scaffolds from bioresorbable polymers, including an electrospun poly(glycolic acid) scaffold for repair of skin cancer excisions and other acute wounds, and another containing encapsulated photoactive antimicrobial agents for repair and management of oral surgical wounds.

Equally, there are some excellent new materials being developed within UK universities. Research in Professor Sandra Downes’ group at the University of Manchester, UK, is focusing on the fabrication of a novel fibrous membrane to address age-related macular degeneration of the eye. This work is still in its early stages, however, assuming fibrous structures can be created to meet the criteria identified (including mechanical robustness to ease handling by surgeons). This could be of great interest in the future.

At Imperial College, a group led by Professor Molly Stevens is investigating the design of a bioactive osteochondral construction for the treatment of osteoarthritis. Because existing scaffold materials exhibit largely isotropic structures, the challenge lies in mimicking articular cartilage, which is anisotropic. Electrospinning offers such a possibility, and functionalised scaffolds are being developed to enhance their biocompatibility and, therefore, resemble the native articular cartilage.

UK development
While the UK is developing its nanofibres industry, going forward the primary challenge lies in the development of scaling-up and manufacturing processes that will enable the technology’s full potential to be realised. Concern has been expressed that the UK does not have the infrastructure to produce electrospun nanofibre material in sufficient volume for product and business development, resulting in novel ideas from UK research heading overseas rather than being manufactured in the UK. However, efforts within the UK are being made to reduce this export. An initiative led by Bob Stevens, Professor of Smart Materials and Devices at Nottingham Trent University, UK, has invested in new, controlled environment electrospinning systems and post-processing facilities, specifically designed to help both academics and industry create value from their research and products. Two pilot-scale production tools are available – one is set up to produce 1mx0.7m sheets of either non-woven or aligned fibre, while the other is more flexible – an effective ‘plug and play’ approach, where the users build their own electrospinning SpinPod that fits into the fully interlocked high-voltage system in the laboratory. In this way, multiple users can be accommodated without fear of cross-contamination between trials run by different users. Post-processing tools have centred on extreme UV lasers for cutting and shaping nanofibre fabrics to form bespoke microscale fibrous components. Additionally, UK Government has established the Catapult centres (including high-value manufacturing and cell therapy) to give SMEs access to high-cost capital tools and skilled resources.

At first glance, nanofibres to nanocomposites may be considered a specialist sector. But increasingly, the added value offered by electrospinning in enabling the combination of nanomaterials and specialised molecules to form manageable, functional nanofibre fabrics, is generating interest from product developers across virtually all industry sectors. The NanoKTN will continue to work and assist this sector, reporting on the progress, determining the key challenges and investigating opportunities by engaging with leaders in the field to encourage the creation of new products and services that rely on electrospinning in their supply chain.

Electrospinning
Electrospinning converts polymeric solutions into micro and nanofibrous materials. The solvent-polymer solutions with controlled viscosity, conductivity and volatility are contained in a syringe and mounted on a precision pump, then pumped to a nozzle held at a high electric potential. When the solution reaches the exit aperture of the nozzle, charge separation occurs, creating a charged layer at the surface of the solution. The electrostatic repulsion force competes with the surface tension and when this force is greater, the electrospinning solution forms a liquid cone and a jet of electrically charged liquid is emitted from the apex of the cone. The solution in the jet evaporates, which reduces the crosssection of the jet, increases viscosity and charge density, and cools the solution. Surface charge density increases as the jet moves away from the nozzle until the required surface charge density is reached. To reduce charge density, a rapid electrostatically driven extension occurs that reduces the diameter and promotes entanglement of long-chain polymer molecules. This forms a long, continuous nanofibre that is electrostatically pulled to a rotating collector. Over time, this forms a nanofibrous sheet. Right: Uniform fibres of poly-L-lactic acid (PLLA) at 500x, 1,000x and 2,000x magnification. Fibres are electrospun into non-woven biomimetic scaffolds to support 3D growth of cells. PLLA is a biodegradable medical-grade polymer facilitating translation of research findings into clinical use.

 

Below (from L-R): Uniform fibres of poly-L-lactic acid (PLLA) at 500x, 1,000x and 2,000x magnification. Fibres are electrospun into non-woven biomimetic scaffolds to support 3D growth of cells. PLLA is a biodegradable medical-grade polymer facilitating translation of research findings into clinical use.

 


 

For further information, please contact:
Professor Bob Stevens Professor in Smart Materials and Devices at the
College of Arts and Science, School of Science & Technology,
Nottingham Trent University, bob.stevens@ntu.ac.uk

Dr Barry Park Theme Manager of Chemical and Consumer Products at the NanoKTN, barry.park@nanoktn.com

Dr Felicity Sartain Theme Manager of Healthcare and Life Sciences at the NanoKTN, felicity.sartain@nanoktn.com.