Living print

Newspapers may be on their way out – as print media move online – but paper and printing have an exciting future in biotechnology. Researchers are using printing technology to make bioactive surfaces – paper, plastic and glass coated with ‘bioinks’ containing antibodies, enzymes or DNA. These functionalised surfaces capture biological analytes from blood, soil and water, and can be made into cheap, disposable sensors for the healthcare, food safety and environmental industries.

According to Tim Claypole, director of the Welsh Centre for Printing & Coating, the printing of biological materials has potential as a ‘major disruptive technology’. Printing biomaterials could lead to high volume manufacture of disposable sensors for point of care and home use with low capital cost, using a variety of bioinks printed on different surfaces for a wide range of applications.

However, there are challenges in turning enzymes and antibodies into bioinks. ‘The molecule must survive the printing process,’ Claypole says. ‘Many are sensitive to shear stress and temperature.’ At the Welsh Centre of Coating and Printing, which is involved in a number of projects with Swansea’s new Centre for NanoHealth (CNH), researchers have been looking at screen, inkjet, aerosol, pad and flexography printing. ‘It is a challenge to match the process to the molecule,’ Claypole adds. He predicts that the screen press will be the ‘workhorse’ of the new biological printing industry, allowing any ink to be printed on any surface.

Countries with an established paper industry are particularly interested in bioactive surfaces. In Canada, for example, McMaster University heads the SENTINEL Bioactive Paper Network, composed of 11 universities and seven industrial partners from the printing, ink and paper sectors. The network wants to take the applications of paper to a new level, says scientific director Robert Pelton. ‘We already have paper that can protect us from pathogens through filtration or antimicrobial coatings. Now we want to make paper that can detect pathogens. We are trying to take some of the low hanging fruit from the biomedical world and see if we can get it to work on paper. Our vision is to have biosensors be as prevalent as printed barcodes on paper.’

While the challenges of selectivity and sensitivity are shared by all biosensors, some issues are specific to the combination of paper and biomolecules. Proteins, including enzymes, tend to fall apart when aged, dried out or exposed to sunlight. The SENTINEL researchers are working on four groups of biomolecules to try to overcome these limitations: enzymes; antibodies derived from llamas, which are more stable than human antibodies and which are genetically modified engineered to have a cellulose binding domain to bind to paper; DNA aptamers, which are more stable than regular DNA; and bacteriophages, which are viruses that can detect and kill bacteria.

‘The challenge is that we are trying to do this without equipment,’ says Pelton. ‘Reporting the sensor’s result is the hardest part of the project. But it may be that colour reactions on paper can be linked to cell phones, which are everywhere and generally have a high resolution camera built in. Phones can be connected to a computer which would create a network. There is a lot of excitement over integrating phones with paper diagnostics and sensors, although the technical and business challenges are big ones.’

While paper tests are not new, this is a field that is being ‘rediscovered and pushed forward,’ according to John Brennan of McMaster University. ‘Paper has a lot of unique properties – it is scaleable and disposable. Our idea is to take assays now done on plastic and bring them onto paper - and also to discover new paper-based assays.’ In a recent paper (Analytical Chemistry, 2009, 81, (13), 5474), Brennan and his team described a paper-based biosensor for acetylcholinesterase inhibitors, which are wellknown neurotoxins and a component of bioterror agents and organophosphorus pesticides.

The enzyme acetylcholinesterase was deposited on the paper in a sol-gel formulation of biocompatible silica particles by inkjet printing using equipment from SENTINEL partner Fujifilm Dimatix. The system detected two acetylcholinesterase inhibitors, paraoxon and aflatoxin, with high sensitivity within five minutes with a standard colour reaction and the results were reproducible after storage at 4°C for at least 60 days, making it suitable for use in the field. Aflatoxins are present in some food moulds, so the test paper has a potential role in food safety as well as homeland security and environmental monitoring.