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Note 7 - Stockholders' Equity and Common Stock Warrants - Warrants Outstanding to Acquire Shares of Its Common Stock (Details) - $ / shares | Dec. 31, 2020 | May 31, 2020 | Dec. 31, 2019 | Nov. 30, 2019
--- | --- | --- | --- | ---
Common stock warrants, outstanding (in shares) | 9,100,112 |  |  |
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 |  |  |
Range of exercise price (in dollars per share) |  |  |  |
Warrants Issued in Connection with the January 2018 Offering [Member] | Minimum [Member] |  |  |  |
Range of exercise price (in dollars per share) | 12 |  |  |
Warrants Issued in Connection with the January 2018 Offering [Member] | Maximum [Member] |  |  |  |
Range of exercise price (in dollars per share) | $ 15 |  |  |
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 |  |  |
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) | 497,140 |  |  |
Range of exercise price (in dollars per share) |  |  |  | $ 0.35
Warrants Issued in Connection with the November 2019 Offering [Member] | Minimum [Member] |  |  |  |
Range of exercise price (in dollars per share) | $ 0.35 |  |  |
Warrants Issued in Connection with the November 2019 Offering [Member] | Maximum [Member] |  |  |  |
Range of exercise price (in dollars per share) | $ 0.4375 |  |  |
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) |  |  | $ 0.4335 |
Warrants Issued in Connection with the December 2019 Offering [Member] | Minimum [Member] |  |  |  |
Range of exercise price (in dollars per share) | 0.4335 |  |  |
Warrants Issued in Connection with the December 2019 Offering [Member] | Maximum [Member] |  |  |  |
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) | $ 1.31 | $ 1.3125 |  |
The May 2020 Investor Warrant Exercise [Member] |  |  |  |
Common stock warrants, outstanding (in shares) | 4,250,000 |  |  |
The May 2020 Investor Warrant Exercise [Member] | Minimum [Member] |  |  |  |
Range of exercise price (in dollars per share) | $ 0.5263 |  |  |
The May 2020 Investor Warrant Exercise [Member] | Maximum [Member] |  |  |  |
Range of exercise price (in dollars per share) | $ 0.5938 |  |  |

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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=109259400&loc=d3e21475-112644 + Details
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