One problem with human-animal tissue contact is the enhanced opportunity for zoonosis (cross-species diseases).<p>In the early 1990&#x27;s there was a proposal to enclose pig Islets of Langerhans (insulin and glucagon producing) tissue in capsules with sub-cellular pores and implant them in diabetics.  The idea was that rejection was less likely without direct cellular contact and the diabetics would gain something resembling natural blood-sugar regulation.<p>There were two problems that persist over the three decades of subsequent research: 1. rejection does still prove to be a problem, and 2. the problem was that pig viruses emitted by the transplanted capsules might eventually find an &quot;error&quot; that would infect the human host resulting in a brand new zoonosis shared by pigs and humans. The FDA refused trials as a result (see the last paragraph of this section in wikipedia: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Pancreatic_islets#Transplantation" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Pancreatic_islets#Transplantat...</a> - that article leaves out is the distinct possibility of other porcine viruses that might cross-species given the persistent opportunity this therapy might provide).<p>If one is being conservative - always a good idea when the prospect of new diseases are in the offing - the process of putting human tissues in pigs and then back in humans is at least as risky as merely putting live pig tissue directly into humans.