Note 9 - Options and Warrants to Purchase Common Stock - Summary of the Status of the Options Outstanding (Details)	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Options outstanding, number outstanding (in shares) | shares	59,825,123
Options outstanding, weighted average remaining contractual life (Year)	7 years 40 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.35
Options exercisable, number exercisable (in shares) | shares	44,958,434
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.44
Exercise Price 1 [Member]
Options outstanding, number outstanding (in shares) | shares	34,475,000
Options outstanding, weighted average remaining contractual life (Year)	9 years 255 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.06
Options exercisable, number exercisable (in shares) | shares	22,808,336
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.06
Exercise Price 2 [Member]
Options outstanding, number outstanding (in shares) | shares	545,289
Options outstanding, weighted average remaining contractual life (Year)	9 years 266 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.075
Options exercisable, number exercisable (in shares) | shares	211,955
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.075
Exercise Price 3 [Member]
Options outstanding, number outstanding (in shares) | shares	3,000,000
Options outstanding, weighted average remaining contractual life (Year)	4 years 313 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.12
Options exercisable, number exercisable (in shares) | shares	249,999
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.12
Exercise Price 4 [Member]
Options outstanding, number outstanding (in shares) | shares	500,000
Options outstanding, weighted average remaining contractual life (Year)	3 years 138 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.16
Options exercisable, number exercisable (in shares) | shares	500,000
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.16
Exercise Price 5 [Member]
Options outstanding, number outstanding (in shares) | shares	81,395
Options outstanding, weighted average remaining contractual life (Year)	5 years 324 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.21
Options exercisable, number exercisable (in shares) | shares	81,395
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.21
Exercise Price 6 [Member]
Options outstanding, number outstanding (in shares) | shares	100,000
Options outstanding, weighted average remaining contractual life (Year)	2 years 171 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.22
Options exercisable, number exercisable (in shares) | shares	100,000
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.22
Exercise Price 7 [Member]
Options outstanding, number outstanding (in shares) | shares	1,066,155
Options outstanding, weighted average remaining contractual life (Year)	3 years 43 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.24
Options exercisable, number exercisable (in shares) | shares	1,066,155
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.24
Exercise Price 8 [Member]
Options outstanding, number outstanding (in shares) | shares	2,087,500
Options outstanding, weighted average remaining contractual life (Year)	4 years 197 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.25
Options exercisable, number exercisable (in shares) | shares	2,087,500
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.25
Exercise Price 9 [Member]
Options outstanding, number outstanding (in shares) | shares	35,595
Options outstanding, weighted average remaining contractual life (Year)	5 years 14 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.27
Options exercisable, number exercisable (in shares) | shares	35,595
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.27
Exercise Price 10 [Member]
Options outstanding, number outstanding (in shares) | shares	474,815
Options outstanding, weighted average remaining contractual life (Year)	6 years 40 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.28
Options exercisable, number exercisable (in shares) | shares	474,815
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.28
Exercise Price 11 [Member]
Options outstanding, number outstanding (in shares) | shares	234,506
Options outstanding, weighted average remaining contractual life (Year)	4 years 324 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.285
Options exercisable, number exercisable (in shares) | shares	234,506
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.285
Exercise Price 12 [Member]
Options outstanding, number outstanding (in shares) | shares	81,522
Options outstanding, weighted average remaining contractual life (Year)	2 years 295 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.30
Options exercisable, number exercisable (in shares) | shares	81,522
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.30
Exercise Price 13 [Member]
Options outstanding, number outstanding (in shares) | shares	200,000
Options outstanding, weighted average remaining contractual life (Year)	6 years 321 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.66
Options exercisable, number exercisable (in shares) | shares	200,000
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.66
Exercise Price 14 [Member]
Options outstanding, number outstanding (in shares) | shares	150,000
Options outstanding, weighted average remaining contractual life (Year)	6 years 313 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.68
Options exercisable, number exercisable (in shares) | shares	150,000
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.68
Exercise Price 15 [Member]
Options outstanding, number outstanding (in shares) | shares	7,233,277
Options outstanding, weighted average remaining contractual life (Year)	273 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.70
Options exercisable, number exercisable (in shares) | shares	7,233,277
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.70
Exercise Price 16 [Member]
Options outstanding, number outstanding (in shares) | shares	488,356
Options outstanding, weighted average remaining contractual life (Year)	7 years 47 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.73
Options exercisable, number exercisable (in shares) | shares	371,666
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.73
Exercise Price 17 [Member]
Options outstanding, number outstanding (in shares) | shares	3,104,653
Options outstanding, weighted average remaining contractual life (Year)	3 years 328 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.83
Options exercisable, number exercisable (in shares) | shares	3,104,653
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.83
Exercise Price 18 [Member]
Options outstanding, number outstanding (in shares) | shares	975,000
Options outstanding, weighted average remaining contractual life (Year)	6 years 73 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 0.84
Options exercisable, number exercisable (in shares) | shares	975,000
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.84
Exercise Price 19 [Member]
Options outstanding, number outstanding (in shares) | shares	300,000
Options outstanding, weighted average remaining contractual life (Year)	5 years 142 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 1.10
Options exercisable, number exercisable (in shares) | shares	300,000
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 1.10
Exercise Price 20 [Member]
Options outstanding, number outstanding (in shares) | shares	300,000
Options outstanding, weighted average remaining contractual life (Year)	5 years 87 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 1.15
Options exercisable, number exercisable (in shares) | shares	300,000
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 1.15
Exercise Price 21 [Member]
Options outstanding, number outstanding (in shares) | shares	115,000
Options outstanding, weighted average remaining contractual life (Year)	2 years 361 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 1.35
Options exercisable, number exercisable (in shares) | shares	115,000
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 1.35
Exercise Price 22 [Member]
Options outstanding, number outstanding (in shares) | shares	300,000
Options outstanding, weighted average remaining contractual life (Year)	4 years 54 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 1.55
Options exercisable, number exercisable (in shares) | shares	300,000
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 1.55
Exercise Price 23 [Member]
Options outstanding, number outstanding (in shares) | shares	3,077,060
Options outstanding, weighted average remaining contractual life (Year)	4 years 233 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 1.66
Options exercisable, number exercisable (in shares) | shares	3,077,060
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 1.66
Exercise Price 24 [Member]
Options outstanding, number outstanding (in shares) | shares	900,000
Options outstanding, weighted average remaining contractual life (Year)	3 years 142 days
Options outstanding, weighted average exerice price (in dollars per share) | $ / shares	$ 1.90
Options exercisable, number exercisable (in shares) | shares	900,000
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 1.90