We have developed a digital human model, called VIP-Man, from segmented and labeled Visible Human (Male) image set containing about 100 radio-sensitive organs and tissues (selected from 1400 anatomical structures). The voxel size is 0.33mm x 0.33mm x 1.0 mm and the whole-body contains about 3.7 billion voxels. The VIP-Man model has been coupled with various Monte Carlo codes to simulate occupational and medical scenarios. More recently, a preliminary study has developed a 4D version of the VIP-Man model to account for the motions of the heart and lungs. This paper discusses the past experience and future issues to be addressed including the extension of the existing model into multi-scale testbeds for deformable organ geometry modeling, medical imaging, radiation treatment planning, and virtual surgical simulations.