1791
Natural structures tend to comprise of curves and organic shapes as they represent a balance between minimal energy expenditure and material used on the one hand , and maximal return of work on the other hand , all within the organismdevelopmental limits .
 material: as; machanical property: energy expenditure; material: material; concept or principle: limits
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:
 material: as; machanical property: energy expenditure; material: material; concept or principle: limits