Note 10 - Net Loss Per Share (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Notes Tables

Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block]

	September 30,
	2019		2018
Stock options to purchase common stock		848,148		616,567
Unvested restricted stock units		62,135		29,706
Common stock warrants		2,435,808		-
Total		3,346,091		646,273

	December 31,
	2018		2017		2016
Stock options to purchase common stock		5,477,364		4,213,100		3,491,937
Unvested restricted stock units		267,359		97,996		-
Common stock warrants		2,666,666		-		-
Total		8,411,389		4,311,096		3,491,937