David Bennett Jr. knelt at his bedside, phone in hand, anxiously waiting for the call he’d never received before: The hospital was supposed to update him on whether his father, who had received a new heart transplanted from a pig, was still alive.

It was the first time a living human had received a pig organ transplant.

“I don’t know what the news is, but my dad opened his eyes, he was awake and he was OK. It was unbelievable,” Bennett Jr. said.

Bennett’s father, David Bennett Sr., had severe congestive heart failure and was not a candidate for a transplant. He knew he would likely die soon. There was nothing else he could do but take a chance on a novel, cutting-edge procedure. Bennett Sr. and his son agreed it was worth the risk.

The achievement made headlines around the world following the transplant in January 2022. Initially, the results looked promising, with some family members beginning to entertain the idea that Bennett Sr. might eventually be released from the hospital.

“There were definitely future-oriented conversations about the home environment, who was going to care for him and what that was going to look like,” Bennett Jr. said. “Everyone was very optimistic and hopeful.”

But two months later, Bennett Sr.’s body rejected the heart and he died at age 57. paperDoctors at the University of Maryland Medical Center said his body likely produced too many antibodies to fight the new organ. The drugs he was given may have also increased the chance of rejection, and a virus in the pig’s heart further complicates things.

Three other patients have followed in Bennett Sr.’s footsteps and received pig organs, most recently a pig kidney transplant in April. Together, they are pioneers in the burgeoning field of xenotransplantation. For them, the journey has been a roller coaster of emotions, from anxiety to blind hope and ultimately praise for their loved one’s decision, three family members told NBC News.

“Obviously, I wish my dad was still here, but I know his sacrifice was not in vain,” Bennett Jr. said.

None of the patients survived more than three months. To the public, it may have seemed a failure. But to their families, the transplants had accomplished a goal: to buy their loved ones more time and to advance research that may one day save their lives.

“Larry thought: He’s going to die. It’s inevitable, it’s coming,” says Anne Fawcett, whose husband of nearly 38 years, Laurence Fawcett, is the second person to receive a pig heart transplant. “So to gather as much data as we can, to do as much research as we can, why not use Larry’s body as a test subject, to give people in the future who need a transplant another option?”

The potential of xenotransplantation lies in the shortage of available human organs. 17 people die every day in the United States while waiting for an organ transplantBecause pig organs are more readily available, doctors expect such surgeries to become as common as hip replacements in the future, according to the Health Resources and Services Administration.