In recent years, significant efforts have been made to develop the implantable applications. The increasing functions, performances, and operating time push up the power requirement of the implantable medical microsystems. This has led to the interest in the wireless power transfer (WPT) as a promise way to provide more energy. However, the WPT suffers from some problems including unsatisfied power efficiency, limited transfer distance, unpredictable reliability, thermal issues and other problems. Therefore, this paper investigate in-depth the design challenges of the wireless power transfer for medical microsystems. Both the design concerns at the primary and secondary sides are introduced in-depth. At last, this paper looks in the future and summaries several possible future challenges.