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Note 6 - Stockholders' Equity and Common Stock Warrants - Warrants Outstanding to Acquire Shares of Its Common Stock (Details) - $ / shares | Dec. 31, 2021 | Feb. 28, 2021 | May 31, 2020
--- | --- | --- | ---
Common stock warrants, outstanding (in shares) | 6,499,469 |  |
Warrants Attached to Series A Convertible Preferred Stock [Member] |  |  |
Common stock warrants, outstanding (in shares) | 903,870 |  |
Range of exercise price (in dollars per share) | $ 33.30 |  |
Warrants Issued in Connection with the January 2018 Offering [Member] |  |  |
Common stock warrants, outstanding (in shares) | 1,181,421 |  |
Warrants Issued in Connection with the January 2018 Offering [Member] | Minimum [Member] |  |  |
Range of exercise price (in dollars per share) | $ 12.00 |  |
Warrants Issued in Connection with the January 2018 Offering [Member] | Maximum [Member] |  |  |
Range of exercise price (in dollars per share) | $ 15.00 |  |
Warrants Issued in Connection with May 2019 Public Offering [Member] |  |  |
Common stock warrants, outstanding (in shares) | 1,382,913 |  |
Warrants Issued in Connection with May 2019 Public Offering [Member] | Minimum [Member] |  |  |
Range of exercise price (in dollars per share) | $ 5.00 |  |
Warrants Issued in Connection with May 2019 Public Offering [Member] | Maximum [Member] |  |  |
Range of exercise price (in dollars per share) | $ 6.11875 |  |
Warrants Issued in Connection with the November 2019 Offering [Member] |  |  |
Common stock warrants, outstanding (in shares) | 213,570 |  |
Range of exercise price (in dollars per share) | $ 0.35 |  |
Warrants Issued in Connection with the December 2019 Offering [Member] |  |  |
Common stock warrants, outstanding (in shares) | 313,339 |  |
Range of exercise price (in dollars per share) | $ 0.6981 |  |
The May 2020 Offering Warrants [Member] |  |  |
Common stock warrants, outstanding (in shares) | 571,429 |  |
Range of exercise price (in dollars per share) | $ 1.3125 |  | $ 1.3125
The May 2020 Investor Warrant Exercise [Member] |  |  |
Common stock warrants, outstanding (in shares) | 250,000 |  |
Range of exercise price (in dollars per share) | $ 0.5938 |  |
Underwriter Warrants in Connection with the February 2021 Common Stock Offering [Member] |  |  |
Common stock warrants, outstanding (in shares) | 1,682,927 |  |
Range of exercise price (in dollars per share) | $ 1.28 | $ 1.28125 |

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- Definition Exercise price per share or per unit of warrants or rights outstanding. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=123467817&loc=d3e21475-112644 + Details
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