Note 10 - Summary of Stock Options - Summary of Option Activity Under All Plans (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 $ / shares shares
Options outstanding (in shares) | shares	4,014,475
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 4.56
Options outstanding, weighted average remaining contractual term (Year)	8 years 219 days	7 years 146 days
Options granted (in shares) | shares	2,335,000
Options granted, weighted average exercise price (in dollars per share) | $ / shares	$ 1.18
Options exercised (in shares) | shares
Options exercised, weighted average exercise price (in dollars per share) | $ / shares
Options canceled (in shares) | shares	(1,035,100)
Options canceled, weighted average exercise price (in dollars per share) | $ / shares	$ 4.94
Options outstanding (in shares) | shares	5,314,375	4,014,475
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 3	$ 4.56
Options outstanding, aggregate intrinsic value | $
Vested and exercisable and expected to vest, end of period (in shares) | shares	5,054,030
Vested and exercisable and expected to vest, end of period, weighted average exercise price (in dollars per share) | $ / shares	$ 3.04
Vested and exercisable and expected to vest, end of period, weighted average remaining contractual term (Year)	8 years 219 days
Vested and exercisable and expected to vest, end of period, aggregate intrinsic value | $
Vested and exercisable, end of period (in shares) | shares	1,790,963
Vested and exercisable, end of period, weighted average exercise price (in dollars per share) | $ / shares	$ 4.26
Vested and exercisable, end of period, weighted average remaining contractual term (Year)	7 years 219 days
Vested and exercisable, end of period, aggregate intrinsic value | $