"We have been unable to prove that any crimes have been committed," said public prosecutor Jennie Nordin in a statement.
Macchiarini had been suspected of involuntary manslaughter and of causing grievous bodily harm.
In 2011, Macchiarini was recruited as a top researcher at the Karolinska Institute, one of the world's most prestigious medical universities whose professors annually select the winner of the Nobel Prize in Physiology or Medicine.
Macchiarini also performed surgeries at the Karolinska University Hospital's two campuses in Stockholm. He performed the world's first synthetic organ transplant, replacing a patient's trachea, or windpipe, with a plastic tube seeded with stem cells taken from the patient.
Macchiarini's procedure promised to reshape organ transplantation as it meant patients would not have to wait for a donor organ and run the risk of biological rejection. Instead, the idea was that tracheas – and possibly other organs and body parts – would simply be replaced with new, artificial and made-to-order plastic versions.
However, what was supposed to be a groundbreaking medical procedure turned into a fatal disaster for most patients and now represents what has been described as the biggest medical scandal in modern times in Sweden.
Of the nine patients that received the treatment, in Sweden and elsewhere, only two survived and eventually had their synthetic tracheas removed and replaced with donor windpipes. However, at least one of those two patients later died, too, at a US hospital.
Three of Macchiarini's synthetic trachea operations took place at the Karolinska University Hospital, with the other patients treated at hospitals in Barcelona, Florence, London, Moscow, Krasnodar, Chicago and Peoria.
Last year, an independent report sharply criticised the operations at the Karolinska University Hospital and a separate investigation at the Karolinska Institute identified mistakes made when Macchiarini was recruited and when allegations of misconduct were made against him in 2014.
Since the procedure was radically new, it has been argued that Macchiarini and his colleagues should have tested it on animals first, but they did not. They also failed to undertake a proper risk assessment of the procedure, and Macchiarini's team did not seek government permits for the plastic windpipes, stem cells, and chemical "growth factors" that they used. Neither did the team seek the approval of Stockholm's ethical review board, which is based at Karolinska.
In January 2016, Swedish Television aired a three-part exposé of Macchiarini and his work. The series suggested that Macchiarini's artificial windpipes had done more harm than good. The TV programme’s findings and claims ran contrary to the numerous scientific articles, press releases and interviews that Macchiarini and others had published about his work.
What followed then was a public relations disaster for the Karolinska Institute, which first promised to investigate the allegations and then, soon after, announced that Macchiarini's contract would not be extended.
Prior to today's announcement from the Swedish prosecutors, Macchiarini was suspected of three cases of involuntary manslaughter and of causing grievous bodily harm in a case that involved another kind of operation, not related to the plastic trachea procedure. However, the research fraud allegations are a separate matter being tried by Sweden’s ethical review board. The board is expected to reach a decision soon.
Macchiarini, on his part, has maintained that the operations were necessary and life-saving and represented the only chance of survival for the patients. He has also said that he still believes in his own methods. Regarding the research fraud, he has stressed that he was cleared of suspicions by Karolinska in 2015.
In total Macchiarini, was involved in 29 medical procedures at the Karolinska University Hospital, including the three trachea transplants where all patients have died.
Radio Sweden will be publishing an audio segment with further details from today's press conference this afternoon.
Macchiarini scandal and the impact on the Nobel3:27 min 3:27 min
Karolinska Hospital fires bosses over Macchiarini scandal2:11 min 2:11 min