Note 9 - Convertible Preferred Stock, Stockholders' Equity and Common Stock Warrants (Tables)	12 Months Ended
	Dec. 31, 2018
Notes Tables
Schedule of Warrants Outstanding to Acquire Shares of Common Stock [Table Text Block]	<div style="display: inline; font-family: times new roman; font-size: 10pt"><table border="0" cellpadding="0" cellspacing="0" style="; font-size: 10pt; font-family: "Times New Roman", Times, serif; text-indent: 0px; min-; min-width: 700px;"> <tr style="vertical-align: bottom;"> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 56%;"> </td> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 1%;"> </td> <td colspan="2" style="text-align: center; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; border-bottom: 1px solid rgb(0, 0, 0); width: 1%;"> <div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;"><div style="display: inline; font-weight: bold;"><div style="display: inline; font-weight: bold;"><div style="display: inline; font-weight: bold;"><div style="display: inline; font-weight: bold;">Outstanding</div></div></div></div></div> </td> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; padding-bottom: 1px; width: 1%;"> </td> <td colspan="6" rowspan="1" style="text-align: center; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; border-bottom: 1px solid rgb(0, 0, 0); width: 55%;"> <div style=" font-family: "Times New Roman", Times, serif; font-size: 10pt; margin: 0pt;"><div style="display: inline; font-weight: bold;"><div style="display: inline; font-weight: bold;"><div style="display: inline; font-weight: bold;"><div style="display: inline; font-weight: bold;">Range of exercise</div></div></div></div></div> <div style=" font-family: "Times New Roman", Times, serif; font-size: 10pt; margin: 0pt;"><div style="display: inline; font-weight: bold;"><div style="display: inline; font-weight: bold;"><div style="display: inline; font-weight: bold;"><div style="display: inline; font-weight: bold;">price per share</div></div></div></div></div> </td> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 1%; border-bottom: 1px solid rgb(0, 0, 0);"> </td> <td colspan="1" style="text-align: center; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; border-bottom: 1px solid rgb(0, 0, 0); width: 12%;"> <div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;"><div style="display: inline; font-weight: bold;"><div style="display: inline; font-weight: bold;"><div style="display: inline; font-weight: bold;"><div style="display: inline; font-weight: bold;">Expiration</div></div></div></div></div> <div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;"><div style="display: inline; font-weight: bold;"><div style="display: inline; font-weight: bold;"><div style="display: inline; font-weight: bold;"><div style="display: inline; font-weight: bold;">dates</div></div></div></div></div> </td> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; padding-bottom: 1px; width: 1%;"> </td> </tr> <tr style="vertical-align: bottom; background-color: rgb(204, 238, 255);"> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 56%;"> <div style=" font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-top: 0pt; margin-bottom: 0pt;">Common stock warrants issued prior to 2016</div> </td> <td style="width: 1%; border-bottom: 1px none rgb(0, 0, 0); font-family: "Times New Roman", Times, serif; font-size: 10pt;"> </td> <td style="width: 1%; border-bottom: 1px none rgb(0, 0, 0); font-family: "Times New Roman", Times, serif; font-size: 10pt;"> </td> <td style="width: 12%; border-bottom: 1px none rgb(0, 0, 0); text-align: right; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">1,767</div></td> <td nowrap="nowrap" style="width: 1%; border-bottom: 1px none rgb(0, 0, 0); font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt;"> </td> <td colspan="4" style="text-align: center; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; width: 5%;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div><div style=" font-family: "Times New Roman", Times, serif; font-size: 10pt; margin: 0pt; text-align: right;">$562.50</div> </td> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; width: 5%; text-align: center;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">-</div></td> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; width: 5%;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">$735.00</div></td> <td style="width: 1%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> </td> <td style="width: 12%; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; text-align: center;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">2019</div></td> <td nowrap="nowrap" style="width: 1%; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt;"> </td> </tr> <tr style="vertical-align: bottom; background-color: rgb(255, 255, 255);"> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 56%;"> <div style=" font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-top: 0pt; margin-bottom: 0pt;">Common stock warrants issued in 2017 related to Series A convertible preferred stock offering</div> </td> <td style="width: 1%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> </td> <td style="width: 1%; font-family: "Times New Roman", Times, serif; font-size: 10pt;"> </td> <td style="width: 12%; text-align: right; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">903,870</div></td> <td nowrap="nowrap" style="width: 1%; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt;"> </td> <td colspan="4" rowspan="1" style="width: 5%; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; text-align: right;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div></td> <td rowspan="1" style="width: 5%; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; text-align: center;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">$33.30</div></td> <td rowspan="1" style="width: 5%; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div></td> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 1%;"> </td> <td colspan="1" style="text-align: center; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; width: 12%;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div><div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">March 2022</div> </td> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 1%;"> </td> </tr> <tr style="vertical-align: bottom; background-color: rgb(204, 238, 255);"> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 56%;"> <div style=" font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-top: 0pt; margin-bottom: 0pt;">Common stock warrants issued in 2018 related to the common stock offering</div> </td> <td style="width: 1%; font-family: "Times New Roman", Times, serif; font-size: 10pt; padding-bottom: 1px;"> </td> <td style="width: 1%; font-family: "Times New Roman", Times, serif; font-size: 10pt; border-bottom: 1px solid rgb(0, 0, 0);"> </td> <td style="width: 12%; text-align: right; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; border-bottom: 1px solid rgb(0, 0, 0);"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">1,181,375</div></td> <td nowrap="nowrap" style="width: 1%; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; padding-bottom: 1px;"> </td> <td colspan="4" style="text-align: center; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; width: 5%;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div><div style=" font-family: "Times New Roman", Times, serif; font-size: 10pt; margin: 0pt; text-align: right;">$12.00</div> </td> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; width: 5%; text-align: center;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">-</div></td> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; width: 5%;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">$15.00</div></td> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 1%;"> </td> <td colspan="1" style="text-align: center; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; width: 12%;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div><div style=" font-family:'Times New Roman', Times, serif;font-size:10pt;margin:0pt;text-align:center;">January 2023</div> </td> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 1%;"> </td> </tr> <tr style="vertical-align: bottom; background-color: rgb(255, 255, 255);"> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 56%;"> </td> <td style="width: 1%; font-family: "Times New Roman", Times, serif; font-size: 10pt; padding-bottom: 3px;"> </td> <td style="width: 1%; font-family: "Times New Roman", Times, serif; font-size: 10pt; border-bottom: 3px double rgb(0, 0, 0);"> </td> <td style="width: 12%; text-align: right; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; border-bottom: 3px double rgb(0, 0, 0);"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">2,087,012</div></td> <td nowrap="nowrap" style="width: 1%; font-family: "Times New Roman", Times, serif; font-size: 10pt; margin-left: 0pt; padding-bottom: 3px;"> </td> <td colspan="4" rowspan="1" style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 5%;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;">  </div></td> <td rowspan="1" style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 5%;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div></td> <td rowspan="1" style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 5%;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div></td> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 1%;"> </td> <td style="text-align: right; font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 12%;"><div style="display: inline; font-style: italic; font-weight: inherit; font-style: normal;"> </div></td> <td style="font-family: "Times New Roman", Times, serif; font-size: 10pt; width: 1%;"> </td> </tr> </table></div>