Summary of significant accounting policies and recent accounting pronouncements (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accounting Policies [Abstract]
Advertising expense	$ 8,000	$ 264,000	$ 18,000	$ 772,000
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of product revenue	36,148	179,945	83,477	263,879
Shipping and Handling
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of product revenue	$ 20,000	$ 29,000	$ 35,000	$ 54,000