The technique represents a way to enhance commercially available diagnostic devices that use paper-strip assays, such as those that test for diabetes and pregnancy.
’With current systems that use paper test strips you can measure things such as pH or blood sugar, but you can’t perform more complex chemical assays,’ said Babak Ziaie, a Purdue University professor of electrical and computer engineering and biomedical engineering.
’This new approach offers the potential to extend the inexpensive paper-based systems so that they are able to do more complicated multiple analyses on the same piece of paper. It’s a generic platform that can be used for a variety of applications.’
Current lab-on-a-chip technology is relatively expensive because chips must be specifically designed to perform certain types of chemical analyses, with channels created in glass or plastic and small pumps and valves directing the flow of fluids for testing.
The chips are being used for various applications in medicine and research, measuring specific types of cells and molecules in a patient’s blood, monitoring micro-organisms in the environment and in foods, and separating biological molecules for laboratory analyses. But the chips, which are roughly palm-size or smaller, are complex to design and manufacture.
The new technique is claimed to be simpler because the testing platform will be contained on a disposable paper strip. To create the strip, the researchers start with paper having a hydrophobic — or water-repellent — coating, such as parchment paper or wax paper used for cooking. A laser is used to burn off the hydrophobic coatings in lines, dots and patterns, exposing the underlying water-absorbing paper only where the patterns are formed.
’Since the hydrophobic agent is already present throughout the thickness of the paper, our method creates islands of hydrophilic patterns,’ Ziaie said. ’This modified surface has a highly porous structure, which helps to trap and localise chemical and biological aqueous reagents for analysis. Furthermore, we’ve selectively deposited silica microparticles on patterned areas to allow diffusion from one end of a channel to the other.’
Those microparticles help to wick liquid to a location where it would combine with another chemical, called a reactant, causing it to change colours and indicating a positive or negative test result.
Having a patterned hydrophilic surface is needed for many detection methods in biochemistry, such as enzyme-linked immunosorbent assay, or ELISA, used in immunology to detect the presence of an antibody or an antigen in a sample, Ziaie said.
To demonstrate the new concept, the researchers created paper strips containing arrays of dots dipped in luminol, a chemical that turns fluorescent blue when exposed to blood. The researchers then sprayed blood on the strips, showing the presence of haemoglobin.
Laser modification is known to alter the ’wettability’ of materials by causing structural and chemical changes to surfaces. However, this treatment has never before been done on paper, Ziaie said.
’Our process is much easier because we just use a laser to create patterns on paper you can purchase commercially and it is already impregnated with hydrophobic material,’ he added. ’It’s a one-step process that could be used to manufacture an inexpensive diagnostic tool for the developing world where people can’t afford more expensive analytical technologies.’