Note 10 - Stockholders' Equity - Option Activity for Stock Option Plans (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares
Outstanding, options (in shares) | shares	1,156,027
Outstanding, weighted average exercise price, options (in dollars per share) | $ / shares	$ 3.92
Granted, options (in shares) | shares	2,233,500
Granted, weighted average exercise price, options (in dollars per share) | $ / shares	$ 0.40
Forfeited/cancelled (in shares) | shares	(338,518)
Forfeited/cancelled, weighted average exercise price, options (in dollars per share) | $ / shares	$ 3.10
Expired, options (in shares) | shares	(27,370)
Expired, weighted average exercise price, options (in dollars per share) | $ / shares	$ 4.83
Outstanding, options (in shares) | shares	3,023,639
Outstanding, weighted average exercise price, options (in dollars per share) | $ / shares	$ 1.40
Outstanding, weighted average remaining contractual life, options (Year)	9 years 109 days
Outstanding, aggregate intrinsic value, options | $
Vested and Expected to Vest, options (in shares) | shares	1,893,518
Vested and Expected to Vest, weighted average exercise price, options (in dollars per share) | $ / shares	$ 1.77
Vested and Expected to Vest, weighted average remaining contractual life, options (Year)	8 years 328 days
Vested and Expected to Vest, aggregate intrinsic value, options | $
Exercisable, options (in shares) | shares	901,742
Exercisable, weighted average exercise price, options (in dollars per share) | $ / shares	$ 2.79
Exercisable, weighted average remaining contractual life, options (Year)	8 years 36 days
Exercisable, aggregate intrinsic value, options | $