Note 10 - Summary of Stock Options - Summary of Options Outstanding and Exercisable (Details)	6 Months Ended
	Jun. 30, 2019 $ / shares shares
Options outstanding, number (in shares) | shares	5,314,375
Options outstanding, weighted average exercise price (in dollars per share)	$ 3
Options outstanding, weighted average remaining contractual term (Year)	8 years 233 days
Options exercisable, number exercisable (in shares) | shares	1,790,963
Options exercisable, weighted average exercise price (in dollars per share)	$ 4.26
Range One [Member]
Exercise price range, lower limit (in dollars per share)	0.38
Exercise price range, upper limit (in dollars per share)	$ 0.58
Options outstanding, number (in shares) | shares	190,000
Options outstanding, weighted average exercise price (in dollars per share)	$ 0.53
Options outstanding, weighted average remaining contractual term (Year)	9 years 339 days
Options exercisable, number exercisable (in shares) | shares
Options exercisable, weighted average exercise price (in dollars per share)
Range Two [Member]
Exercise price range, lower limit (in dollars per share)	1
Exercise price range, upper limit (in dollars per share)	$ 1.97
Options outstanding, number (in shares) | shares	2,643,195
Options outstanding, weighted average exercise price (in dollars per share)	$ 1.38
Options outstanding, weighted average remaining contractual term (Year)	9 years 153 days
Options exercisable, number exercisable (in shares) | shares	485,265
Options exercisable, weighted average exercise price (in dollars per share)	$ 1.52
Range Three [Member]
Exercise price range, lower limit (in dollars per share)	2.02
Exercise price range, upper limit (in dollars per share)	$ 2.83
Options outstanding, number (in shares) | shares	85,000
Options outstanding, weighted average exercise price (in dollars per share)	$ 2.48
Options outstanding, weighted average remaining contractual term (Year)	8 years 226 days
Options exercisable, number exercisable (in shares) | shares	20,520
Options exercisable, weighted average exercise price (in dollars per share)	$ 2.65
Range Four [Member]
Exercise price range, lower limit (in dollars per share)	3.03
Exercise price range, upper limit (in dollars per share)	$ 3.82
Options outstanding, number (in shares) | shares	474,376
Options outstanding, weighted average exercise price (in dollars per share)	$ 3.55
Options outstanding, weighted average remaining contractual term (Year)	8 years 200 days
Options exercisable, number exercisable (in shares) | shares	175,002
Options exercisable, weighted average exercise price (in dollars per share)	$ 3.65
Range Five [Member]
Exercise price range, lower limit (in dollars per share)	4.30
Exercise price range, upper limit (in dollars per share)	$ 4.97
Options outstanding, number (in shares) | shares	920,023
Options outstanding, weighted average exercise price (in dollars per share)	$ 4.52
Options outstanding, weighted average remaining contractual term (Year)	8 years 40 days
Options exercisable, number exercisable (in shares) | shares	409,403
Options exercisable, weighted average exercise price (in dollars per share)	$ 4.55
Range Six [Member]
Exercise price range, lower limit (in dollars per share)	5.01
Exercise price range, upper limit (in dollars per share)	$ 5.67
Options outstanding, number (in shares) | shares	510,696
Options outstanding, weighted average exercise price (in dollars per share)	$ 5.37
Options outstanding, weighted average remaining contractual term (Year)	7 years 40 days
Options exercisable, number exercisable (in shares) | shares	317,831
Options exercisable, weighted average exercise price (in dollars per share)	$ 5.37
Range Seven [Member]
Exercise price range, lower limit (in dollars per share)	6
Exercise price range, upper limit (in dollars per share)	$ 6.61
Options outstanding, number (in shares) | shares	242,627
Options outstanding, weighted average exercise price (in dollars per share)	$ 6.03
Options outstanding, weighted average remaining contractual term (Year)	6 years 105 days
Options exercisable, number exercisable (in shares) | shares	209,608
Options exercisable, weighted average exercise price (in dollars per share)	$ 6.01
Range Eight [Member]
Exercise price range, lower limit (in dollars per share)	7
Exercise price range, upper limit (in dollars per share)	$ 7.92
Options outstanding, number (in shares) | shares	238,024
Options outstanding, weighted average exercise price (in dollars per share)	$ 7.70
Options outstanding, weighted average remaining contractual term (Year)	7 years 14 days
Options exercisable, number exercisable (in shares) | shares	162,900
Options exercisable, weighted average exercise price (in dollars per share)	$ 7.71
Range Nine [Member]
Exercise price range, lower limit (in dollars per share)	9.92
Exercise price range, upper limit (in dollars per share)	$ 9.92
Options outstanding, number (in shares) | shares	10,424
Options outstanding, weighted average exercise price (in dollars per share)	$ 9.92
Options outstanding, weighted average remaining contractual term (Year)	3 years 219 days
Options exercisable, number exercisable (in shares) | shares	10,424
Options exercisable, weighted average exercise price (in dollars per share)	$ 9.92
Range Ten [Member]
Exercise price range, lower limit (in dollars per share)	59.60
Exercise price range, upper limit (in dollars per share)	$ 149.04
Options outstanding, number (in shares) | shares	10
Options outstanding, weighted average exercise price (in dollars per share)	$ 149.04
Options outstanding, weighted average remaining contractual term (Year)	2 years 113 days
Options exercisable, number exercisable (in shares) | shares	10
Options exercisable, weighted average exercise price (in dollars per share)	$ 149.04