Note 8 - Derivative Warrant Liability - Schedule of Shares Issued Upon Exercise of Warrants (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018		Feb. 28, 2017	Feb. 17, 2017	Sep. 13, 2016	Apr. 01, 2016	Dec. 11, 2015	May 19, 2015	Feb. 03, 2015	Aug. 05, 2014
Shares underlying warrants (in shares)	55,963	[1]	408,141
Shares issues upon warrant exercises (in shares)	352,178
2014 Warrants [Member]
Warrant, issuance date	Aug. 05, 2014
Warrants, expiration date	Aug. 05, 2019
Warrant exercise price (in dollars per share)	$ 65.50
Shares underlying warrants (in shares)	974	[1]								2,500
Shares issues upon warrant exercises (in shares)	1,526
Series A Warrants [Member]
Warrant, issuance date	Feb. 03, 2015
Warrants, expiration date	Feb. 03, 2020
Warrant exercise price (in dollars per share)	$ 3.80
Shares underlying warrants (in shares)	320	[1]							5,542
Shares issues upon warrant exercises (in shares)	5,222
Series C Warrants [Member]
Warrant, issuance date	May 19, 2015
Warrants, expiration date	May 19, 2020
Warrant exercise price (in dollars per share)	$ 53.43
Shares underlying warrants (in shares)	1,075	[1]						1,075
Shares issues upon warrant exercises (in shares)
Series D Warrants [Member]
Warrant, issuance date	Dec. 11, 2015
Warrants, expiration date	Dec. 11, 2020
Warrant exercise price (in dollars per share)	$ 40
Shares underlying warrants (in shares)	47	[1]					25,125
Shares issues upon warrant exercises (in shares)	25,078
Series F Warrants [Member]
Warrant, issuance date	Apr. 01, 2016
Warrants, expiration date	Apr. 01, 2021
Warrant exercise price (in dollars per share)	$ 40
Shares underlying warrants (in shares)	14,041	[1]				25,733
Shares issues upon warrant exercises (in shares)	11,692
Series I Warrants [Member]
Warrant, issuance date	Sep. 13, 2016
Warrants, expiration date	Sep. 13, 2021
Warrant exercise price (in dollars per share)	$ 220
Shares underlying warrants (in shares)	35,650	[1]			35,650
Shares issues upon warrant exercises (in shares)
Series K Warrants [Member]
Warrant, issuance date	Feb. 17, 2017
Warrants, expiration date	Feb. 17, 2022
Warrant exercise price (in dollars per share)	$ 3.80
Shares underlying warrants (in shares)	3,856	[1]		312,516
Shares issues upon warrant exercises (in shares)	308,660

[1] This table does not include equity-classified warrants issued in 2011, to purchase 6 shares of common stock with a strike price of $345 per share.