Note 10 - Options and Warrants to Purchase Common Stock - Summary of the Status of the Options Outstanding (Details)	12 Months Ended
	Dec. 31, 2016 $ / shares shares
Options outstanding, weighted average remaining contractual life (Year)	4 years 219 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.87
Options exercisable (in shares) | shares	20,104,136
Options exercisable, weighted average exercise price (in dollars per share)	$ 0.87
Exercise Price 1 [Member]
Options outstanding, weighted average remaining contractual life (Year)	9 years 189 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.16
Options exercisable (in shares) | shares	166,667
Options exercisable, weighted average exercise price (in dollars per share)	$ 0.16
Exercise Price 2 [Member]
Options outstanding, weighted average remaining contractual life (Year)	7 years 51 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.21
Options exercisable (in shares) | shares	0
Options exercisable, weighted average exercise price (in dollars per share)
Exercise Price 3 [Member]
Options outstanding, weighted average remaining contractual life (Year)	3 years 259 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.22
Options exercisable (in shares) | shares	100,000
Options exercisable, weighted average exercise price (in dollars per share)	$ 0.22
Exercise Price 4 [Member]
Options outstanding, weighted average remaining contractual life (Year)	4 years 131 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.24
Options exercisable (in shares) | shares	1,066,155
Options exercisable, weighted average exercise price (in dollars per share)	$ 0.24
Exercise Price 5 [Member]
Options outstanding, weighted average remaining contractual life (Year)	6 years 273 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.25
Options exercisable (in shares) | shares	1,297,500
Options exercisable, weighted average exercise price (in dollars per share)	$ 0.25
Exercise Price 6 [Member]
Options outstanding, weighted average remaining contractual life (Year)	6 years 105 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.27
Options exercisable (in shares) | shares	0
Options exercisable, weighted average exercise price (in dollars per share)
Exercise Price 7 [Member]
Options outstanding, weighted average remaining contractual life (Year)	7 years 131 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.28
Options exercisable (in shares) | shares	250,000
Options exercisable, weighted average exercise price (in dollars per share)	$ 0.28
Exercise Price 8 [Member]
Options outstanding, weighted average remaining contractual life (Year)	6 years 47 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.285
Options exercisable (in shares) | shares	234,506
Options exercisable, weighted average exercise price (in dollars per share)	$ 0.285
Exercise Price 9 [Member]
Options outstanding, weighted average remaining contractual life (Year)	4 years 18 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.30
Options exercisable (in shares) | shares	0
Options exercisable, weighted average exercise price (in dollars per share)
Exercise Price 10 [Member]
Options outstanding, weighted average remaining contractual life (Year)	8 years 43 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.66
Options exercisable (in shares) | shares	200,000
Options exercisable, weighted average exercise price (in dollars per share)	$ 0.66
Exercise Price 11 [Member]
Options outstanding, weighted average remaining contractual life (Year)	8 years 36 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.68
Options exercisable (in shares) | shares	49,995
Options exercisable, weighted average exercise price (in dollars per share)	$ 0.68
Exercise Price 12 [Member]
Options outstanding, weighted average remaining contractual life (Year)	1 year 361 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.70
Options exercisable (in shares) | shares	7,233,277
Options exercisable, weighted average exercise price (in dollars per share)	$ 0.70
Exercise Price 13 [Member]
Options outstanding, weighted average remaining contractual life (Year)	8 years 138 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.73
Options exercisable (in shares) | shares	255,011
Options exercisable, weighted average exercise price (in dollars per share)	$ 0.73
Exercise Price 14 [Member]
Options outstanding, weighted average remaining contractual life (Year)	5 years 54 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.83
Options exercisable (in shares) | shares	3,104,653
Options exercisable, weighted average exercise price (in dollars per share)	$ 0.83
Exercise Price 15 [Member]
Options outstanding, weighted average remaining contractual life (Year)	7 years 164 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.84
Options exercisable (in shares) | shares	891,667
Options exercisable, weighted average exercise price (in dollars per share)	$ 0.84
Exercise Price 16 [Member]
Options outstanding, weighted average remaining contractual life (Year)	6 years 233 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 1.10
Options exercisable (in shares) | shares	300,000
Options exercisable, weighted average exercise price (in dollars per share)	$ 1.10
Exercise Price 17 [Member]
Options outstanding, weighted average remaining contractual life (Year)	6 years 175 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 1.15
Options exercisable (in shares) | shares	300,000
Options exercisable, weighted average exercise price (in dollars per share)	$ 1.15
Exercise Price 18 [Member]
Options outstanding, weighted average remaining contractual life (Year)	1 year 29 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 1.24
Options exercisable (in shares) | shares	100,000
Options exercisable, weighted average exercise price (in dollars per share)	$ 1.24
Exercise Price 19 [Member]
Options outstanding, weighted average remaining contractual life (Year)	4 years 87 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 1.35
Options exercisable (in shares) | shares	115,000
Options exercisable, weighted average exercise price (in dollars per share)	$ 1.35
Exercise Price 20 [Member]
Options outstanding, weighted average remaining contractual life (Year)	1 year 29 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 1.45
Options exercisable (in shares) | shares	125,000
Options exercisable, weighted average exercise price (in dollars per share)	$ 1.45
Exercise Price 21 [Member]
Options outstanding, weighted average remaining contractual life (Year)	4 years 346 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 1.55
Options exercisable (in shares) | shares	330,000
Options exercisable, weighted average exercise price (in dollars per share)	$ 1.55
Exercise Price 22 [Member]
Options outstanding, weighted average remaining contractual life (Year)	1 year 244 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 1.58
Options exercisable (in shares) | shares	7,645
Options exercisable, weighted average exercise price (in dollars per share)	$ 1.58
Exercise Price 23 [Member]
Options outstanding, weighted average remaining contractual life (Year)	5 years 324 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 1.66
Options exercisable (in shares) | shares	3,077,060
Options exercisable, weighted average exercise price (in dollars per share)	$ 1.66
Exercise Price 24 [Member]
Options outstanding, weighted average remaining contractual life (Year)	4 years 229 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 1.90
Options exercisable (in shares) | shares	900,000
Options exercisable, weighted average exercise price (in dollars per share)	$ 1.90