In a quiet, husky voice (he has yet another painful lung infection), he made light of the physiotherapy he has to undergo to clear his lungs of mucus. 'My girlfriend beats me up twice a day and makes sure I take my tablets,' he said.
He has known Dawn Wooton for 14 months and she hopes to be a nurse if her exam results this week are good enough. She visits him daily to help, and has applied for an invalidity attendance allowance so that she can continue.
Now 23, Mr Keen lives in a flat on his own in Hemel Hempstead and has been unable to work for two years because of his illness. Since he was two, his life has been dominated by cystic fibrosis. He has to take 70 tablets a day to stop him getting chest infections, to improve his breathing, and to help him digest food. This is supplemented by nebulisers and surfactant, a lubricant for the surface of the lungs.
After many years of fighting lung infections, his breathing was laboured as he talked. Mr Keen has a tube into a vein in his arm for the high doses of antibiotics he needs.
When doctors at the National Heart and Lung Hospital in Kensington told him about five months ago about the possibility of the treatment, his reaction was: 'I'll be cured.' However, he said that now that he understood the therapy better, he did not expect a cure. 'But it might stop me getting an infection. I'm pleased to be able to help.'
The hospital has the largest clinic for treating cystic fibrosis in Europe. But neither doctors nor the nine volunteers will know until November, at the earliest, whether the technique has worked.