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For the MEX SCF part , the average pore area was found to be highest ( > 250 μm2 ) at the bottom of the layer and smallest ( < 100 μm2 ) at the top of the layer , which could be explained by a large temperature gradient between and contractile thermal stresses inside the layer that cause the thermoplastic to shrink into a smaller volume allowing the voids to grow during deposition .
 concept or principle: average; process parameter: area; material: be; process parameter: layer; process parameter: layer; material: be; process parameter: temperature gradient; machanical property: thermal stresses; process parameter: layer; material: thermoplastic; engineering feature: shrink; concept or principle: volume; concept or principle: voids; concept or principle: deposition
Given options, please tell me the categories of all the listed entity words.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} 
 Entities: average, area, be, layer, layer, be, temperature gradient, thermal stresses, layer, thermoplastic, shrink, volume, voids, deposition 
Answer:
 concept or principle: average; process parameter: area; material: be; process parameter: layer; process parameter: layer; material: be; process parameter: temperature gradient; machanical property: thermal stresses; process parameter: layer; material: thermoplastic; engineering feature: shrink; concept or principle: volume; concept or principle: voids; concept or principle: deposition