Note 14 - Subsequent Events (Details Textual) - USD ($) $ / shares in Units, $ in Thousands			6 Months Ended
	Jul. 06, 2020	Mar. 10, 2020	Jun. 30, 2020	Jun. 30, 2019	Jul. 31, 2020	Apr. 28, 2017
Shares Issued, Price Per Share (in dollars per share)						$ 1.00
Proceeds from Issuance of Equity Net			$ 13,525	$ 0
Series B Warrant [Member]
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)			$ 0.37			$ 13.00
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)			530,381
Subsequent Event [Member] | Restricted Cash Held with Silicon Valley Bank [Member]
Restricted Cash, Total					$ 500
Subsequent Event [Member] | Silicon Valley Bank [Member]
Letters of Credit Outstanding, Amount					$ 500
Subsequent Event [Member] | Office, Lab and Warehouse Space in Durham, North Carolina [Member]
Lessee, Operating Lease, Lease Not yet Commenced, Term of Contract (Month)					125 months
Lessee, Operating Lease, Lease Not yet Commenced, Renewal Term (Year)					5 years
Lessee, Operating Lease, Liability, to be Paid, Total					$ 5,000
Subsequent Event [Member] | Series B Warrant [Member]
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)	$ 0.35
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)	567,660
Public Offering [Member]
Proceeds from Issuance of Equity Net		$ 13,500
Public Offering [Member] | Subsequent Event [Member]
Stock Issued During Period, Shares, New Issues (in shares)	42,857,142
Shares Issued, Price Per Share (in dollars per share)	$ 0.35
Proceeds from Issuance of Equity Net	$ 13,700