Northstar Neuroscience hired an investment bank to evaluate a buyout proposal by the Seattle-based medical-device company's largest shareholder.

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Given a one-week deadline from its largest shareholder to consider a buyout offer, Northstar Neuroscience said Wednesday it has hired an investment bank to assess its alternatives.

Tang Capital Partners of San Diego, which said it owns 18 percent of Northstar, went public Wednesday with a $2.25-per-share proposal to take the company private. News of the bid sent Northstar stock up 34 cents, or 22.7 percent, to close at $1.84.

The offer represents a 50 percent premium over the closing price of Northstar’s stock on Tuesday. Tang Capital said the proposal is contingent on receiving a positive response by July 9.

Northstar’s stock has traded between $13.39 and $1.09 in the past 52 weeks, plunging in January after the company announced the failure of a clinical trial evaluating its cortical stimulation to improve hand and arm function in stroke survivors.

Tang Capital said in a filing with the Securities and Exchange Commission that it has “tried on several occasions to have constructive, confidential discussions with management” regarding a buyout offer.

“So far, management has been unwilling to entertain such discussions, except under conditions that would make it impossible for us to protect the value of our significant investment in Northstar,” it said.

The offer values Northstar’s 25.9 million shares at approximately $58.2 million, though Tang would be paying only about $47.7 million for the 82 percent it doesn’t already own. As of March 31, the company reported it had cash and investments of $73 million.