2019
The other useful development is the optimization of repeated lattices gradient lattices and variations of strut thickness or unit cell size across a part , and conformal lattices to the surfaces of a part .
 concept or principle: optimization; concept or principle: lattices; concept or principle: lattices; concept or principle: variations; process parameter: strut thickness; concept or principle: unit cell; engineering feature: conformal lattices; concept or principle: surfaces
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
Answer:
 concept or principle: optimization; concept or principle: lattices; concept or principle: lattices; concept or principle: variations; process parameter: strut thickness; concept or principle: unit cell; engineering feature: conformal lattices; concept or principle: surfaces