Condensed Balance Sheets (Current Period Unaudited) (Parentheticals) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Accounts receivable, allowance for doubtful accounts	$ 6	$ 19
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares issued (in shares)	52,276	52,369
Preferred stock, shares outstanding (in shares)	52,276	52,369
Preferred stock, liquidation preference	$ 52,191	$ 52,191
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	95,562,955	84,926,129
Common stock, shares outstanding (in shares)	95,562,955	84,926,129