Eventually, Karin Hwang wants to reengineer injured human brains. But right now she’s intent on launching her first invention, a device for use by expectant mothers that detects signs the baby may arrive too early. Hwang, 23, earned a master’s degree in biomedical engineering from Johns Hopkins University last May, entering one of the few sizzling parts of the job market. Positions in her chosen field are expected to grow by 72 percent between 2008 and 2018, far outpacing the 11 percent average for all engineering fields.