18545
In this review , we discuss some of the progress that has recently been made in the development of Gd - based macromolecules and nanoparticles and outline some of the physical and chemical properties that will be important to incorporate into the next generation of contrast agents , including high Gd chelate stability , high " relaxivity per particle " and " relaxivity density " , and biodegradability .
 Chemical: Gd; Chemical: Gd
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
 Entities: Gd, Gd 
Answer:
 Chemical: Gd; Chemical: Gd