Many Hip plants are metal  which requires  extra care during surgical implantation and can cause problems if parts  grind against each other  and release particle  that  may cause inflammation  and damage organs.  Now  researchers have   developed a  metal-free alternative  a  special blend of plastics.  The pins that old  the two pieces in place  are made of  bone mineral (not metal or cement)   In  early  tests a  robot simulated  walking up and down stairs. The  hip may market  within yeas.