Note: the schema.org specification states that the about property is expected to be an item of type Thing. That means we should mark up each subject as the name of a Thing or some subtype of Thing, in this case it makes sense to class subject as Intangible for example:
    <span itemprop="about"     itemscope itemtype="http://schema.org/Intangible">
        <span itemprop="name">Material Science</span>
    </span>
This is shown in figure 5, but in the example above we have made use of the short cut described in the the schema.org documentation:
We also expect that often, where we expect a property value of type Person, Place, Organization or some other subClassOf Thing, we will get a text string. In the spirit of "some data is better than none", we will accept this markup and do the best we can.
http://schema.org/docs/datamodel.html