DataSource Controls :: Map Stringbuilder With Varchar?

Im developing a web application in asp.net where i have gridview with some text box controls in it.In runtime we are creating a new row dynamically by clicking add button .To insert data into the database.i used a stringbuilder instead of string becoz the data what i have to insert into database is more.but i am gettin the following error No mapping exists from object type System.Text.StringBuilder to a known managed provider native type. in database the datafield column datatype is varchar(max).