After staying mostly on the sidelines of merger and acquisition activity last year, Stryker Corp. said it would be an active buyer and put its strong balance sheet to work this year. Representing the most recent step in that direction, the company agreed to shell out $1.28 billion for Physio-Control, a portfolio company of Bain Capital Private Equity that makes monitors and defibrillators, automated external defibrillators (AEDs) and CPR-assist devices along with offering data management and support services.