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

6 Months Ended

Jun. 30, 2020

Notes Tables

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

	Outstanding		Range of exercise price per shar			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	November 2024
Common stock warrants issued related to the November 2019 offering		514,283	$0.35	-	$0.4375	May 2024
Common stock warrants issued related to the December 2019 offering		313,339		$0.6981		December 2024
Common stock warrants issued related to the May 2020 offering		571,429		$1.3125		May 2025
Common stock warrants issued related to May 2020 investor warrant exercise		4,250,000	$0.5263	-	$0.5938	November 2025
		9,117,209