But devices like ecg machines are expensive , hard to find and require expertise - especially in the developing world .
但类似于心电图机的仪器是十分昂贵的，而且对于普通人他很难使用也不易找到在发展中国家尤其如此。
As a proof of concept , lin 's system currently takes the user 's ecg reading from each palm once , and a key based on that reading is stored and used for all later decryptions .
The devices were small and relatively inexpensive and they were originally " developed for markets in emerging economies ( the ecg device for rural india and the ultrasound machine for rural china ) and are now being sold in the us " .
