This seems a case where you want to compose a string from several elements.
You can do this with the string datatype (as you have done here). The benefit is that this will automagically convert numbers to string before concatenating the elements if you use +, while char will be treated as a number if you use +.
Another option is to move to sprintf. That way you have control over the exact format used.
app.model = "some_name";
app.ID = 3;
statementUpdate = "UPDATE acquisition_table SET model = '" + app.model + "' WHERE ID = 3 "
statementUpdate = sprintf('UPDATE acquisition_table SET model = ''%s'' WHERE ID = %d',app.model,app.ID)
As you can see, both return the same thing, just a different datatype.
One last thing: I thought you should provide the SQLite string datatype with " instead of ' (and that you need to finish every statement with a semicolon). That might cause the underlying problem with this statement.
statementUpdate = sprintf('UPDATE acquisition_table SET model = "%s" WHERE ID = %d;',app.model,app.ID)
