MIRRORS could soon reflect the health of cancer patients. Instead of running regular blood tests to see if a tumour is growing back, a doctor will use a simple instrument to bounce a low-power laser beam off a silicon 鈥渕irror鈥� implanted under the patient鈥檚 skin. Computer analysis of the reflected beam will tell you if cancer has returned.
Some cancers, like prostate and colorectal, release specific chemical markers into the bloodstream as they develop. Doctors can spot the disease by looking for these markers. 鈥淛ust one cell of a tumour will shed these markers,鈥� says Stephen Mather, head of nuclear medicine at St Bartholomew鈥檚 Hospital, London. After surgery, or other treatment, cancer patients have blood tests every three to six months, for up to five years, to check for the chemical markers that warn when the tumour is growing back. 鈥淭he amount of marker in the blood is directly related to the size of the tumour,鈥� says Mather.
Now Roger Aston and Leigh Canham of pSiMedica, a biomedical company based in Malvern, Worcestershire, believe they鈥檝e found a better way to pick up early signs of cancer. They want to put mirrors under the skin that change their reflectivity as the cancer marker builds up in the bloodstream. You can check the reflectivity by firing an infrared beam through the skin at the mirror and measuring the reflected light, says Aston. 鈥淩ather than having to go for regular blood tests, you could just drop by your doctor and get a quick, non-invasive check,鈥� he says.
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Getting the optical properties of the mirror right is crucial, says Canham. Light only passes easily through skin over a tight range of infrared wavelengths, but silicon surfaces don鈥檛 reflect well at these wavelengths. And if the mirror was highly polished it would quickly lose its finish under the skin because it鈥檚 designed to break down. But by making the silicon sheet porous it will reflect light at the right wavelengths, even if the surfaces have heavily degraded.
Canham controlled the etching process of the silicon mirrors-which are 5 millimetres wide and 0.5 millimetres thick-to build up a series of layers of alternating high and low porosity. This gives alternate layers different refractive indices, so a beam of light hitting the mirror will be reflected at the interfaces between neighbouring layers. These beams interfere with each other to give a reflected beam whose wavelength can be tuned by changing the porosity and thickness of the layers, says Canham. 鈥淎nd it doesn鈥檛 matter if you lose layers through degradation- it still functions as a mirror deeper down in the material.鈥�
The researchers found that the mirrors slowly broke down into harmless silicic acid in simulated human blood plasma. Their next step will be to fill the pores of the mirror with chemicals that bind to specific cancer markers. Aston says that markers in the bloodstream would leach into tissues around the implant and bind to the chemicals in the mirror鈥檚 pores. 鈥淚f you then shine a light on it, the reflected beam will change depending on how much of the marker you鈥檝e got,鈥� says Aston.
鈥淚t sounds a nice idea, but they鈥檒l have to show they can get a good signal out of this when it starts binding to the markers,鈥� says Mather. 鈥淚t鈥檒l also be interesting to see if they can make this more sensitive than the current blood tests.鈥�
