Medical device giant Medtronic plans to buy Framingham-based HeartWare International in a $1.1 billion deal that nearly doubles HeartWare’s market value as of Friday.

HeartWare makes a device called the HVAD heart pump, a mechanical pump that can be implanted in people with advanced heart failure when one ventricle of the heart does not work properly. The company has been developing a smaller version of the same pump, called the MVAD, which has not launched yet.

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