Note 9 - Stockholders' Equity (Details Textual) $ / shares in Units, xbrli-pure in Thousands				1 Months Ended		3 Months Ended	6 Months Ended
	Jun. 30, 2020 USD ($) $ / shares shares	Apr. 06, 2020 USD ($) $ / shares shares	May 14, 2019 $ / shares shares	Feb. 28, 2019 USD ($) $ / shares shares	May 31, 2014 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2020 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2019 $ / shares shares	Dec. 17, 2019 shares	Dec. 31, 2018 $ / shares shares
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares	$ 1.95					$ 1.48	$ 1.95	$ 1.48	$ 0.50		$ 2.55
Proceeds from Issuance of Private Placement | $				$ 250,000	$ 10,000,000			$ 250,000
Share Price (in dollars per share) | $ / shares						$ 0.36		$ 0.36
Proceeds from Issuance of Common Stock | $							$ 18,003,037	$ 1,974,547
Deferred Compensation Arrangement with Individual, Shares Authorized for Issuance (in shares)	2,519,630					2,903,366	2,519,630	2,903,366	2,375,760		2,470,565
Common Stock, Shares, To be Issued (in shares)	2,900,000						2,900,000
Derivative Instruments Not Designated as Hedging Instruments, Gain | $						$ 12,500		$ 52,700
Common Stock, Shares Authorized (in shares)	75,000,000						75,000,000		75,000,000	75,000,000
Derivative, Number of Instruments Held, Total	0						0
Chief Executive Officer, Chief Financial Officer and Employees [Member]
Deferred Compensation Arrangement with Individual, Shares Authorized for Issuance (in shares)	2,370,345						2,370,345		2,185,910
Non-Employees [Member]
Common Stock, Capital Shares Reserved for Future Issuance (in shares)	149,285					717,456	149,285	717,456
Preferred Stock Converted to Common Stock [Member]
Conversion of Stock, Conversion Price (in dollars per share) | $ / shares			$ 1.17
Conversion of Stock, Shares Issued (in shares)			5,982,906
Warrants, Public Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)				1,428,750
Warrants and Rights Outstanding, Term (Year)				5 years
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares				$ 0.50
Underwrite Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)				141,850
Warrants, Private Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)				178,571
Warrants and Rights Outstanding, Term (Year)				5 years
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares				$ 0.50
Warrants Associated With April 2020 Public Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)		2,710,000
Warrants and Rights Outstanding, Term (Year)		3 years
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares		$ 1.20
Warrants Associated With the June 2020 Public Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)	3,749,000						3,749,000
Warrants and Rights Outstanding, Term (Year)	3 years						3 years
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares	$ 2.60						$ 2.60
Public Offering [Member]
Proceeds from Issuance or Sale of Equity, Total | $				$ 2,000,000
Stock Issued During Period, Shares, New Issues (in shares)				5,715,000
Proceeds from Stock Options Exercised | $				$ 198,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period (in shares)				567,400
Private Placement [Member]
Stock Issued During Period, Shares, New Issues (in shares)				714,286	2,000,000
Proceeds from Issuance of Common Stock | $					$ 3,000,000
Shares Issued, Price Per Share (in dollars per share) | $ / shares					$ 1.50
Proceeds from Issuance of Convertible Preferred Stock | $					$ 7,000,000
Private Placement [Member] | Series A Convertible Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)					7,000
Shares Issued, Price Per Share (in dollars per share) | $ / shares					$ 1,000
Preferred Stock, Redemption Price Per Share (in dollars per share) | $ / shares					2.545
Private Placement [Member] | Series A Convertible Preferred Stock [Member] | Conversion Price if Common Stock Not Trade at 3.15 [Member]
Preferred Stock, Redemption Price Per Share (in dollars per share) | $ / shares					$ 1.50
April 6, 2020 Public Offering [Member]
Stock Issued During Period, Shares, New Issues (in shares)		5,420,000
Proceeds from Issuance of Common Stock, Gross | $		$ 5,100,000
Share Price (in dollars per share) | $ / shares		$ 0.95
Proceeds from Issuance of Common Stock | $		$ 4,600,000
June 30, 2020 Public Offering [Member]
Stock Issued During Period, Shares, New Issues (in shares)	6,770,000
Proceeds from Issuance of Common Stock, Gross | $	$ 14,600,000
Share Price (in dollars per share) | $ / shares	$ 2.15						$ 2.15
Proceeds from Issuance of Common Stock | $	$ 13,400,000