Note 9 - Stockholders' Equity (Details Textual) | May 30, 2018USD ($)$ / sharesshares | Dec. 20, 2017USD ($)$ / sharesshares | Oct. 26, 2017USD ($)$ / sharesshares | Jun. 29, 2017$ / sharesshares | Apr. 03, 2017USD ($)$ / shares$ / itemshares | Jun. 30, 2018USD ($) | Jun. 30, 2018USD ($) | Dec. 31, 2018USD ($)$ / sharesshares | Dec. 31, 2017USD ($)$ / sharesshares | May 29, 2018$ / sharesshares | May 09, 2018$ / itemshares | Sep. 30, 2017shares | Dec. 31, 2016$ / sharesshares | May 19, 2014$ / sharesshares |
Number of Shares Authorized to Issue | | | | | | | | 185,000,000 | | | | | | |
Common Stock, Shares Authorized | | | | | | | | 175,000,000 | 175,000,000 | | | | | |
Common Stock, Par or Stated Value Per Share | $ / shares | | | | | | | | $ 0.18 | $ 0.18 | | | | | |
Preferred Stock, Shares Authorized | | | | | | | | 10,000,000 | 10,000,000 | | | | | |
Preferred Stock, Par or Stated Value Per Share | $ / shares | | | | | | | | $ 0.001 | $ 0.001 | | | | | |
Stockholder Rights Agreement, Number of Preferred Stock Right Distributed for Each Share of Common Stock | | | | | | | | | | | | | | 1 |
Stockholder Rights Agreement, Acquiring Person, Percentage of Ownership | | | | | | | | | | | | | | 15.00% |
Stockholder Rights Agreement, Acquiring Person, Additional Percentage of Ownership | | | | | | | | | | | | | | 2.00% |
Stockholder Rights Agreement, Initial Exercise Price of Each Right | $ / shares | | | | | | | | | | | | | | $ 15 |
Stockholder Rights Agreement, Share Price | $ / shares | | | | | | | | | | | | | | $ 30 |
Stock Issued During Period, Shares, New Issues | | | | 125,000 | | | | | | | | | | |
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares | | | | | | | | | $ 0 | | | | $ 0 | |
Preferred Stock, Deemed Dividend | $ | | | | | | $ 11,479,308 | $ 11,479,308 | $ 11,479,308 | $ 2,568,132 | | | | | |
Lock-up Period Extension | | | | 45 days | | | | | | | | | | |
Class of Warrant or Right, Outstanding | | | | | | | | 3,875,699 | 0 | | | | 0 | |
Proceeds from Issuance of Stock units | $ | | | | | | | | | $ 3,871,636 | | | | | |
Consummate a Business Combination, Percentage of Acquired Outstanding Shares | | | | | | | | 50.00% | | | | | | |
Consummate a Business Combination, Percentage of Aggregate Ordinary Voting Power | | | | | | | | 50.00% | | | | | | |
Conversion of Series A Convertible Preferred Stock into Common Stock [Member] | | | | | | | | | | | | | | |
Conversion of Stock, Shares Converted | | | | | | | | | 292 | | | | | |
Stock Issued During Period, Shares, Conversion of Convertible Securities | | | | | | | | | 389,111 | | | | | |
Convertible Preferred Stock, Shares Issued upon Conversion | | | | | | | | | 111 | | | | | |
Conversion of Series B Convertible Preferred Stock into Common Stock [Member] | | | | | | | | | | | | | | |
Conversion of Stock, Shares Converted | | | | | | | | 11,245 | | | | | | |
Convertible Preferred Stock, Shares Issued upon Conversion | | | | | | | | 284 | | | | | | |
Conversion of Stock, Shares Issued | | | | | | | | 3,194,600 | | | | | | |
The 2017 Public Offering Class B Units Warrants [Member] | | | | | | | | | | | | | | |
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares | | | | $ 3.12 | | | | | | | | | | |
Class of Warrant or Right Cancelled | | | | 250,000 | | | | | | | | | | |
Class of Warrant or Right, Outstanding | | | | | | | | | 0 | | | 0 | | |
The 2017 Public Offering Class B Units Warrants [Member] | Weighted Average [Member] | | | | | | | | | | | | | | |
Share Price | $ / shares | | | | $ 6.24 | | | | | | | | | | |
The 2018 Warrants [Member] | | | | | | | | | | | | | | |
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares | | | | | | | | $ 4.048 | | | | | | |
Warrants and Rights Outstanding, Term | | | | | | | | 4 years | | | | | | |
Class of Warrant or Right, Outstanding | | | | | | | | 3,869,216 | | | | | | |
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right | | | | | | | | 1 | | | | | | |
Warrants Exercisable, Ownership Percentage of Company’s Shares Allowed | | | | | | | | 4.99% | | | | | | |
Common Stock, Convertible, Ownership Percentage of Company’s Shares Allowed | | | | | | | | 9.99% | | | | | | |
Warrants and Rights Outstanding, Redemption Price | $ / shares | | | | | | | | $ 0.18 | | | | | | |
Warrants and Rights Outstanding, Redeemable, Volume-weighted-average-price of Our Common Stock | $ / shares | | | | | | | | $ 10.56 | | | | | | |
Warrants and Rights, Redemption Rights, Consecutive Trading Days | | | | | | | | 10 | | | | | | |
The 2017 Public Offering [Member] | | | | | | | | | | | | | | |
Proceeds from Issuance or Sale of Equity, Total | $ | | | | | $ 4,400,000 | | | | | | | | | |
Proceeds from Issuance or Sale of Equity, Net | $ | | | | | $ 3,900,000 | | | | | | | | | |
Stock Issued During Period, Shares, New Issues | | | | | 55,333 | | | | | | | | | |
Adjustments to Additional Paid in Capital, Beneficial Conversion Feature on Preferred Stock | $ | | | | | $ 1,284,066 | | | | | | | | | |
The 2017 Public Offering [Member] | The 2017 Public Offering Class A Units Warrants [Member] | | | | | | | | | | | | | | |
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | | | | | 55,333 | | | | | | | | | |
The 2017 Public Offering [Member] | The 2017 Public Offering Class B Units Warrants [Member] | | | | | | | | | | | | | | |
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | | | | | 389,111 | | | | | | | | | |
Over-Allotment Option [Member] | | | | | | | | | | | | | | |
Stock Issued During Period, Shares, New Issues | | | 83,333 | | 44,167 | | | | | | | | | |
Shares Issued, Price Per Share | $ / shares | | | $ 5.28 | | | | | | | | | | | |
Over-Allotment Option [Member] | The 2017 Public Offering Over-allotment Warrants [Member] | | | | | | | | | | | | | | |
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | | | | | 44,167 | | | | | | | | | |
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares | | | | | $ 11.25 | | | | | | | | | |
Warrants and Rights Outstanding, Term | | | | | 5 years | | | | | | | | | |
October 2017 Public Offering [Member] | | | | | | | | | | | | | | |
Proceeds from Issuance or Sale of Equity, Net | $ | | | $ 4,900,000 | | | | | | | | | | | |
Stock Issued During Period, Shares, New Issues | | | 958,333 | | | | | | | | | | | |
Proceeds from Issuance of Common Stock | $ | | | $ 5,500,000 | | | | | | | | | | | |
Shares Issued, Price Per Share | $ / shares | | | $ 5.28 | | | | | | | | | | | |
December 2017 Public Offering and Private Placement [Member] | | | | | | | | | | | | | | |
Proceeds from Issuance or Sale of Equity, Net | $ | | $ 1,200,000 | | | | | | | | | | | | |
Stock Issued During Period, Shares, New Issues | | 441,667 | | | | | | | | | | | | |
Proceeds from Issuance of Common Stock | $ | | $ 1,400,000 | | | | | | | | | | | | |
Shares Issued, Price Per Share | $ / shares | | $ 3.24 | | | | | | | | | | | | |
December 2017 Public Offering and Private Placement [Member] | Class A and Class B Warrants [Member] | | | | | | | | | | | | | | |
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | | 883,333 | | | | | | | | | | | | |
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares | | $ 3.78 | | | | | | | | | | | | |
Warrants and Rights Outstanding, Term | | 1 year | | | | | | | | | | | | |
Class of Warrant or Right, Exercisable Period | | 180 days | | | | | | | | | | | | |
The 2018 Subscription Rights Offering [Member] | | | | | | | | | | | | | | |
Number of Units Sold | 13,624 | | | | | | | | | | | | | |
Purchase Price of Unit | $ / item | | | | | | | | | | | 1,000 | | | |
Adjustments to Additional Paid in Capital, Beneficial Conversion Feature on Preferred Stock | $ | | | | | | | | $ 4,782,100 | | | | | | |
Preferred Stock, Deemed Dividend | $ | | | | | | | | 11,479,308 | | | | | | |
Number of Units Authorized to Purchase, Maximum | | | | | | | | | | | 25,000 | | | |
Number of Series B Convertible Preferred Stock and Warrant in Each Unit | | | | | | | | | | | 1 | | | |
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right | | | | | | | | | | | 284 | | | |
Proceeds from Issuance of Stock units | $ | $ 12,300,000 | | | | | | | | | | | | | |
Payments of Stock Issuance Costs | $ | | | | | | | | 1,333,449 | | | | | | |
The 2018 Subscription Rights Offering [Member] | The 2018 Warrants [Member] | | | | | | | | | | | | | | |
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares | $ 4.048 | | | | | | | | | | | | | |
Class of Warrant or Right, Outstanding | 3,869,216 | | | | | | | | | | | | | |
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right | 1 | | | | | | | | | | | | | |
Warrants Allocated to Common Stock , Net of Issuance Cost | $ | | | | | | | | $ 5,363,759 | | | | | | |
Series A Junior Participating Preferred Stock [Member] | | | | | | | | | | | | | | |
Preferred Stock, Shares Authorized | | | | | | | | 750,000 | | | | | | |
Preferred Stock, Par or Stated Value Per Share | $ / shares | | | | | | | | $ 0.001 | | | | | | |
Preferred Stock, Shares Issued, Total | | | | | | | | 0 | | | | | | |
Preferred Stock, Shares Outstanding, Ending Balance | | | | | | | | 0 | | | | | | |
Series A Convertible Preferred Stock [Member] | | | | | | | | | | | | | | |
Preferred Stock, Shares Authorized | | | | | | | | 4,000 | 4,000 | | | | | |
Preferred Stock, Par or Stated Value Per Share | $ / shares | | | | | | | | $ 0.001 | $ 0.001 | | | | | |
Preferred Stock, Shares Issued, Total | | | | | | | | 0 | | | | | | |
Preferred Stock, Deemed Dividend | $ | | | | | $ 2,568,132 | | | | | | | | | |
Preferred Stock, Shares Outstanding, Ending Balance | | | | | | | | 0 | 0 | | | | | |
Series A Convertible Preferred Stock [Member] | The 2017 Public Offering [Member] | | | | | | | | | | | | | | |
Stock Issued During Period, Shares, New Issues | | | | | 292 | | | | | | | | | |
Convertible Preferred Stock, Shares Issuable | | | | | 389,111 | | | | | | | | | |
Series B Convertible Preferred Stock [Member] | | | | | | | | | | | | | | |
Preferred Stock, Shares Authorized | | | | | | | | 25,000 | 0 | 25,000 | | | | |
Preferred Stock, Par or Stated Value Per Share | $ / shares | | | | | | | | $ 0.001 | $ 0.001 | $ 0.001 | | | | |
Preferred Stock, Shares Issued, Total | | | | | | | | 2,379 | 0 | | | | | |
Preferred Stock, Shares Outstanding, Ending Balance | | | | | | | | 2,379 | 0 | | | | | |
Stated Value per Share | $ / shares | | | | | | | | 1,000 | | | | | | |
Preferred Stock, Convertible, Conversion Price | $ / shares | | | | | | | | $ 3.52 | | | | | | |
Preferred Stock, Convertible, Ownership Percentage of Company’s Shares Allowed | | | | | | | | 9.99% | | | | | | |
Series B Convertible Preferred Stock [Member] | The 2018 Subscription Rights Offering [Member] | | | | | | | | | | | | | | |
Stock Issued During Period, Shares, New Issues | 13,624 | | | | | | | | | | | | | |
Class A Units [Member] | The 2017 Public Offering [Member] | | | | | | | | | | | | | | |
Number of Units Sold | | | | | 55,333 | | | | | | | | | |
Purchase Price of Unit | $ / item | | | | | 9 | | | | | | | | | |
Class B Units [Member] | The 2017 Public Offering [Member] | | | | | | | | | | | | | | |
Number of Units Sold | | | | | 292 | | | | | | | | | |
Purchase Price of Unit | $ / item | | | | | 12,000 | | | | | | | | | |