Note 9 - Stockholders' Equity (Details Textual)						3 Months Ended	6 Months Ended	12 Months Ended
	May 30, 2018 USD ($) $ / shares shares	Dec. 20, 2017 USD ($) $ / shares shares	Oct. 26, 2017 USD ($) $ / shares shares	Jun. 29, 2017 $ / shares shares	Apr. 03, 2017 USD ($) $ / shares $ / item shares	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	May 29, 2018 $ / shares shares	May 09, 2018 $ / item shares	Sep. 30, 2017 shares	Dec. 31, 2016 $ / shares shares	May 19, 2014 $ / shares shares
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