Note 4 - Stockholders' Equity, Stock Options and Warrants - Schedule of Listing of Stock Options and Warrants (Details)	6 Months Ended
	Jun. 30, 2018 $ / shares shares
Shares, options (in shares) | shares	3,120,255
Shares, warrants (in shares) | shares	2,556,382
Warrant exercises, exercise price per share (in dollars per share)
Minimum [Member]
Price, options (in dollars per share)	0.91
Warrant exercises, exercise price per share (in dollars per share)	0
Maximum [Member]
Price, options (in dollars per share)	596.25
Warrant exercises, exercise price per share (in dollars per share)	$ 609.38
Warrants 2014 [Member]
Shares, warrants (in shares) | shares	6,455
Warrants 2014 [Member] | Minimum [Member]
Warrant exercises, exercise price per share (in dollars per share)	$ 243.75
Warrants 2014 [Member] | Maximum [Member]
Warrant exercises, exercise price per share (in dollars per share)	$ 609.38
Warrants 2015 [Member]
Shares, warrants (in shares) | shares	94,151
Warrants 2015 [Member] | Minimum [Member]
Warrant exercises, exercise price per share (in dollars per share)	$ 0
Warrants 2015 [Member] | Maximum [Member]
Warrant exercises, exercise price per share (in dollars per share)	$ 243.75
Warrants 2016 [Member]
Shares, warrants (in shares) | shares	504,666
Warrant exercises, exercise price per share (in dollars per share)	$ 1
Warrants 2017 [Member]
Shares, warrants (in shares) | shares	1,082,946
Warrants 2017 [Member] | Minimum [Member]
Warrant exercises, exercise price per share (in dollars per share)	$ 1.07
Warrants 2017 [Member] | Maximum [Member]
Warrant exercises, exercise price per share (in dollars per share)	$ 2.25
Warrants 2018 [Member]
Shares, warrants (in shares) | shares	868,164
Warrant exercises, exercise price per share (in dollars per share)	$ 1
Warrants 2018 [Member] | Minimum [Member]
Warrant exercises, exercise price per share (in dollars per share)
Warrants 2018 [Member] | Maximum [Member]
Warrant exercises, exercise price per share (in dollars per share)
Stock Options 2011 [Member]
Shares, options (in shares) | shares	173
Price, options (in dollars per share)	$ 281.25
Stock Options 2012 [Member]
Shares, options (in shares) | shares	1,841
Stock Options 2012 [Member] | Minimum [Member]
Price, options (in dollars per share)	$ 131.25
Stock Options 2012 [Member] | Maximum [Member]
Price, options (in dollars per share)	$ 150
Stock Options 2013 [Member]
Shares, options (in shares) | shares	1,553
Stock Options 2013 [Member] | Minimum [Member]
Price, options (in dollars per share)	$ 148.13
Stock Options 2013 [Member] | Maximum [Member]
Price, options (in dollars per share)	$ 596.25
Stock Options 2014 [Member]
Shares, options (in shares) | shares	836
Stock Options 2014 [Member] | Minimum [Member]
Price, options (in dollars per share)	$ 162.50
Stock Options 2014 [Member] | Maximum [Member]
Price, options (in dollars per share)	$ 431.25
Stock Options 2015 [Member]
Shares, options (in shares) | shares	4,088
Stock Options 2015 [Member] | Minimum [Member]
Price, options (in dollars per share)	$ 65.75
Stock Options 2015 [Member] | Maximum [Member]
Price, options (in dollars per share)	$ 86.25
Stock Options 2016 [Member]
Shares, options (in shares) | shares	100,292
Stock Options 2016 [Member] | Minimum [Member]
Price, options (in dollars per share)	$ 2.25
Stock Options 2016 [Member] | Maximum [Member]
Price, options (in dollars per share)	$ 5.13
Stock Options 2017 [Member]
Shares, options (in shares) | shares	2,529,070
Stock Options 2017 [Member] | Minimum [Member]
Price, options (in dollars per share)	$ 1.01
Stock Options 2017 [Member] | Maximum [Member]
Price, options (in dollars per share)	$ 2.10
Stock Options 2018 [Member]
Shares, options (in shares) | shares	482,402
Stock Options 2018 [Member] | Minimum [Member]
Price, options (in dollars per share)	$ 0.91
Stock Options 2018 [Member] | Maximum [Member]
Price, options (in dollars per share)	$ 1.35