Spontaneously formed metal-based bioparticles, such as those formed in situ and intrinsic particles transformed ex situ, as well as in vivo modified and transformed extrinsic particles, are often present in the biogical media including body fluids, tissues and biointerfaces. The size of these particles are between nano- and micro- meters. Most of them are involved in the physiological, pathological and toxicological processes. Up to now, however, there is a lack of research on the relationship between the biological effects and the formation, structure and properties of these particles. The present article lists some facts and preliminary understanding about these metal-based bioparticles. Some research areas to be investigated are also proposed. The findings in these areas may provide the insight into the studies on the environment, health, disease control and so on.

Contents
1 Introduction
2 Calcium-based bioparticles under physiological conditions
2.1 Calcium phosphate nanoparticles in bone formation and remodeling
2.2 Calcium phosphate nanoparticles in tooth formation
2.3 Calcium-based nanoparticles under physiological conditions
3 Metal-based bioparticles formed in the pathological process
3.1 Nephronic systemic fibrosis and Gd-containing particles
3.2 Renal failure and deposition of metal-based bioparticles
3.3 Vascular calcification and calcific uremic arteriolopathy
3.4 Calcium-based bioparticles in the stone formation of urinary tract
3.5 Basic calcium phosphate in the synovial fluid
3.6 From nanobacteria to calcifying nanoparticle
4 In vivo modificaton of extrinsic nanoparticles and their biological effects
4.1 In vivo modificaton of extrinsic nanoparticles
4.2 Generality and specificity of nanoparticles-induced toxicity
4.3 The dual biological effects of cerium oxide nanoparticles
5 In vitro formation of metal-based bioparticles
5.1 Formation of metal-based bioparticles in the simulated biological fluid
5.2 Cell-involved formation, transport and transformation of bioparticles
5.3 Influence of metal-based bioparticles on gene transfection efficiency
5.4 Alum as a vaccine adjuvant
6 Summary and perspective