Note 5 - Stockholders' Equity and Common Stock Warrants (Tables)

3 Months Ended

Mar. 31, 2020

Notes Tables

Schedule of Stockholders' Equity Note, Warrants or Rights [Table Text Block]

	Outstanding		Range of exercise price per share			Expiration dates
Common stock warrants issued in 2017 related to Series A convertible preferred stock offering		903,870		$33.30		March 2022
Common stock warrants issued in 2018 related to the common stock offering		1,181,375	$12.00	-	$15.00	January 2023
Common stock warrants issued related to the May 2019 Offering		1,382,913	$5.00	-	$6.11875	May and December 2024
Common stock warrants issued related to the November 2019 Offering		11,212,786	$0.35	-	$0.4375	May 2024
Common stock warrants issued related to the December 2019 Offering		6,580,126	$0.4335	-	$0.6981	December 2024 and June 2025
		21,261,070