Note 9 - Warrants (Details Textual) - $ / shares			12 Months Ended
	Mar. 25, 2021	May 14, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	May 31, 2016	Mar. 31, 2014
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)			$ 0	$ 4.75	$ 4.33	$ 4.35
Class of Warrant or Right, Exercised During Period (in shares)			79,100	3,879,905	121,000
Warrants to Purchase Common Stock [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)								6,910,283
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)								$ 4.75
Warrants in Connection with WAVI Credit Facility [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)							550,000
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)							$ 1.75
WAVI Holding AG and Taurus4757 GmbH Warrants [Member]
Stock Issued During Period, Shares, Cashless Warrant Exercises (in shares)		2,747,970
Class of Warrant or Right, Exercised During Period (in shares)		3,871,405	79,100	3,871,405
Warrants Exercised [Member]
Stock Issued During Period, Shares, Cashless Warrant Exercises (in shares)	70,030
Class of Warrant or Right, Exercised During Period (in shares)	79,100