Drug and medical device maker Covidien PLC said Friday it completed its $210 million purchase of Aspect Medical Systems, a company that makes brain monitoring technology.

Covidien said its tender offer for Aspect stock, which was worth $12 per share, closed at midnight. Aspect, which is headquartered in Norwood, Mass., is now a subsidy of Covidien, which is based in Ireland.

Aspect shares were trading at $7.67 when Covidien made its offer, making the company's bid a 56 percent premium. The stock finished at $12.27 on Friday. In aftermarket trading, Aspect shares fell 20 cents to $12.07, and Covidien stock gave up 11 cents to $43.94.