Note 10 - Stockholders' Equity (Details Textual)													1 Months Ended		3 Months Ended		12 Months Ended		28 Months Ended		58 Months Ended	59 Months Ended
	Feb. 06, 2018 USD ($)	Oct. 31, 2017 USD ($)	Oct. 27, 2017 $ / shares shares	Oct. 04, 2017 USD ($) $ / shares shares	Jul. 06, 2017 USD ($) $ / shares shares	Jun. 26, 2017 USD ($) $ / shares shares	Jun. 09, 2017 USD ($) $ / shares shares	May 26, 2017 shares	Feb. 14, 2017 USD ($) $ / shares shares	Dec. 20, 2016 USD ($) $ / shares shares	Jun. 13, 2016 USD ($) $ / shares shares	Feb. 01, 2013 USD ($)	Sep. 30, 2015 USD ($)	Feb. 25, 2013 USD ($)	Mar. 26, 2018 shares	Dec. 31, 2017 $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Feb. 06, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	May 25, 2017 shares	May 16, 2017 shares	May 15, 2017 shares	Jun. 30, 2016 shares	May 31, 2016 shares	Dec. 31, 2015 $ / shares shares	May 27, 2015 $ / shares shares	Jan. 15, 2014 $ / shares shares	Feb. 22, 2013 $ / shares shares
Shelf Registration Statement Amount | $													$ 75,000,000
Common Stock, Shares Authorized																112,500,000	112,500,000	112,500,000			112,500,000	112,500,000				112,500,000	75,000,000
Preferred Stock, Shares Authorized																100,000	100,000	100,000			100,000	100,000				100,000
Aggregate of Common and Preferred Stock, Shares Authorized																										112,600,000
Common Stock, Shares, Outstanding								4,070,172								17,276,965	17,276,965	2,230,118			17,276,965	17,276,965	56,982,418
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number																703,442	703,442	211,213			703,442	703,442		200,000	2,400,000			154,581
Class of Warrant or Right, Outstanding																3,058,402	3,058,402	1,487,958			3,058,402	3,058,402		2,400,000	33,500,000			421,015
Common Stock, Par or Stated Value Per Share | $ / shares																$ 0.01	$ 0.01	$ 0.01			$ 0.01	$ 0.01
Class of Warrant or Right, Number of Securities Called by Warrants or Rights						506,627	790,410
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares						$ 1.65	$ 2.70									$ 5.29	$ 5.29	$ 9.39			$ 5.29	$ 5.29						$ 5,888
Proceeds from Warrant Exercises | $																	$ 21,140,304	$ 2,500		$ 800,000	$ 2,100,000
Aggregate Offering Price, Additions | $																	13,500,000
2013 Warrants, 2014 Warrants, and June 2016 Warrants [Member]
Increase (Decrease) in Fair Value of Warrants | $							$ 200,000
2015 Warrants [Member]
Increase (Decrease) in Fair Value of Warrants | $						$ 100,000
October 2017 Underwritten Offering [Member]
Stock Issued During Period, Shares, New Issues			2,640,000
Common Stock, Par or Stated Value Per Share | $ / shares			$ 0.01
Class of Warrant or Right, Number of Securities Called by Warrants or Rights			1,320,000
Class of Warrant or Right, Number of Warrants Issued with Stock			0.5
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right			1
Class of Warrant or Right, Offered Price of Warrants or Rights | $ / shares			$ 2.50
Shares Issued, Price Per Share | $ / shares			$ 2.325
Class of Warrant or Right, Exercisable Period			180 days
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares			$ 3
Warrants Issued, Expiration Period			5 years
Proceeds from Issuance of Common Stock and Warrants | $		$ 6,600,000
October 2017 Underwritten Offering [Member] | Additional Warrants Issued to Underwriter [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights			66,000
Class of Warrant or Right, Exercisable Period			180 days
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares			$ 2.87
Warrants Issued, Expiration Period			5 years
July 2017 Direct Offering [Member]
Stock Issued During Period, Shares, New Issues					2,050,000
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right				4,665,000
Share Price | $ / shares					$ 2.07
Proceeds from Issuance of Common Stock, Gross | $					$ 4,243,500
Proceeds from Warrant Exercises | $				$ 16,600,000
July 2017 Direct Offering [Member] | Pre-Funded Series CCC Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights					385,000
Class of Warrant or Right, Offered Price of Warrants or Rights | $ / shares					$ 2.06
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares					$ 0.01
Beneficial Ownership Percentage Threshold					9.99%
Proceeds from Issuance of Warrants | $					$ 793,100
Class of Warrant or Right, Exercised During Period																385,000
July 2017 Direct Offering [Member] | Series AAA Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights					2,435,000
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right					1
Class of Warrant or Right, Exercisable Period					180 days
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares					$ 2.07
Warrants Issued, Expiration Period					5 years 182 days
July 2017 Direct Offering [Member] | Series BBB Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights					2,435,000
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares					$ 4.75
Warrants Issued, Expiration Period					1 year
July 2017 Direct Offering [Member] | Series DDD Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights				1,166,250
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right				1
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares				$ 6.20
February 14, 2017 Public Offering [Member]
Stock Issued During Period, Shares, New Issues									1,384,704
Shares Issued, Price Per Share | $ / shares									$ 3.22
Maximum Beneficial Ownership of the Purchaser									9.99%
Proceeds from Issuance of Common Stock | $									$ 5,000,000
February 14, 2017 Public Offering [Member] | Series AA Warrant [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights									1,177,790
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares									$ 3.22
Proceeds from Warrant Exercises | $																	$ 2,400,000
Class of Warrant or Right, Term									5 years
Stock Issued During Period, Shares, Exercise of Warrants																	747,254
February 14, 2017 Public Offering [Member] | Pre-Funded Series BB Warrant [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights									185,713
Shares Issued, Price Per Share | $ / shares									$ 3.08
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares									$ 0.14
Stock Issued During Period, Shares, Exercise of Warrants																	185,713
December 2016 Common Stock Offering [Member]
Stock Issued During Period, Shares, New Issues										367,346
Class of Warrant or Right, Number of Securities Called by Warrants or Rights										367,343
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares										$ 6.44
Warrants Issued, Expiration Period										5 years 182 days
Share Price | $ / shares										$ 4.90
Proceeds from Issuance of Common Stock | $										$ 1,800,000
Warrants Exercisable Period										180 days
June 2016 Common Stock Offering [Member]
Stock Issued During Period, Shares, New Issues											165,126
Common Stock, Par or Stated Value Per Share | $ / shares											$ 0.01
Class of Warrant or Right, Number of Securities Called by Warrants or Rights											630,252
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares											$ 19.60
Share Price | $ / shares											$ 19.04
Proceeds from Issuance of Common Stock | $											$ 6,000,000
June 2016 Common Stock Offering [Member] | Series A Warrant [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right											0.5
Warrants Issued, Expiration Period											5 years 182 days
Warrants Exercisable Period											180 days
June 2016 Common Stock Offering [Member] | Series C Warrant [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right											1
Warrants Issued, Expiration Period											1 year
Warrants Exercisable Period											180 days
June 2016 Common Stock Offering [Member] | Series D Warrant [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right											0.5
Warrants Issued, Expiration Period											5 years 182 days
Warrants Exercisable Period											180 days
February 2013 Preferred Stock Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights																															95,811
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares																															$ 74.34
January 2014 Common Stock Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights																														64,348
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares																														$ 57.40
May 2015 Common Stock Offering [Member] | 2015 Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights																													139,284
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares																													$ 36.40
ATM Agreement [Member]
Stock Issued During Period, Shares, New Issues																	1,221,648					1,327,326
Proceeds from Issuance of Common Stock | $																	$ 3,900,000					$ 11,500,000
Aggregate Offering Price | $												$ 25,000,000					$ 13,500,000
Commission on Proceeds from ATM Shares Percent														3.00%
Legal Fees | $														$ 50,000
ATM Agreement [Member] | Subsequent Event [Member]
Stock Issued During Period, Shares, New Issues															457,070
Aggregate Offering Price, Additions | $	$ 10,000,000																		$ 17,500,000
Reverse Stock Split [Member]
Stockholders' Equity Note, Stock Split, Conversion Ratio								14