Note 9 - Stockholders' Equity (Details Textual) - USD ($)							1 Months Ended		2 Months Ended	12 Months Ended
	Jun. 19, 2019	Nov. 01, 2018	Jul. 16, 2018	Jul. 13, 2018	Feb. 16, 2018	Feb. 14, 2018	Dec. 31, 2019	Jan. 31, 2019	May 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Aug. 28, 2019	Aug. 22, 2018	Dec. 31, 2017	Jan. 31, 2015
Preferred Stock, Shares Authorized							5,000,000			5,000,000	5,000,000
Preferred Stock, Par or Stated Value Per Share					$ 0.001		$ 0.001			$ 0.001	$ 0.001
Preferred Stock, Shares Issued, Total							48,503			48,503	45,671
Debt Conversion, Original Debt, Amount											$ 42,794,000
Preferred Stock, Shares Outstanding, Ending Balance							48,503			48,503	45,671
Proceeds from Issuance or Sale of Equity, Net					$ 15,500,000
Preferred Stock Deemed Dividend Arising from Beneficial Conversion Feature											$ 5,216,000
Common Stock, Shares Authorized							100,000,000			100,000,000	100,000,000
Common Stock, Par or Stated Value Per Share							$ 0.001			$ 0.001	$ 0.001
Common Stock, Shares, Issued, Total							10,364,663			10,364,663	3,492,200
Class of Warrant or Right, Number of Securities Called by Warrants or Rights					1,797,900
Class of Warrant or Right, Outstanding							2,753,999			2,753,999	4,757,539
Proceeds from Warrant Exercises									$ 8,000,000	$ 7,993,000	$ 581,000
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period										4 years
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period										10 years
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period										0
Share-based Payment Arrangement, Expense, Tax Benefit										$ 0	$ 0
Share Price							$ 1.14			$ 1.14
Common Stock, Shares, Outstanding, Ending Balance							10,364,663			10,364,663	3,492,200
Share-based Payment Arrangement, Option [Member]
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total							$ 72,000			$ 72,000
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition										1 year
Restricted Stock Units (RSUs) [Member]
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total							$ 3,000,000			$ 3,000,000
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition										1 year 292 days
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number, Ending Balance							908,504			908,504	294,066			509
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Fair Value							$ 1,000,000			$ 1,000,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value										$ 116,000	$ 1,500
Minimum [Member] | Common Stock Options [ member]
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent										100.00%
Minimum [Member] | Common Stock Options [ member] | Stockholder with More than 10% Voting Power [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent										110.00%
Minimum [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period										3 years
Maximum [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period										4 years
The 2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized															3,300
Share-based Compensation Arrangement by Share-based Payment Award, Additional Shares Reserved for Issuance, Annual															4,225
Share-based Compensation Arrangement by Share-based Payment Award, Additional Shares Reserved for Issuance, Percentage on Shares Outstanding															5.00%
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized	800,000										300,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant							96,896			96,896
Officer and Director Share Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized												40,000	20,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant							20,204			20,204
Stock Issued During Period, Shares, Employee Stock Purchase Plans										36,087
Placement Agency Sale - Prospectus Supplement of July 12, 2018 [Member]
Proceeds from Issuance or Sale of Equity, Net			$ 3,000,000
November Public Offering [Member]
Stock Issued During Period, Shares, New Issues		728,500
Common Stock Warrants, Issued with Series E Convertible Preferred Stock [Member]
Class of Warrant or Right, Securities Called as Percentage of Convertible Preferred Stock Shares							50.00%			50.00%
Common Stock Warrants, Issued with Series B Preferred Stock [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights					1,797,900		1,768,850			1,768,850
Common Stock Warrants, Issued with Series B Preferred Stock, Warrant One [Member]
Class of Warrant or Right, Number of Warrants Issued for Each Share of Preferred Stock					1
Class of Warrant or Right, Exercise Price of Warrants or Rights					$ 4
Class of Warrant or Right, Contingent Expiration Period Following Initial Exercise Date					7 years
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right					50
Common Stock Warrants, Issued with Series B Preferred Stock, Warrant Two [Member]
Class of Warrant or Right, Number of Warrants Issued for Each Share of Preferred Stock					1
Class of Warrant or Right, Contingent Expiration Period Following Initial Exercise Date					7 years
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right					50
Class of Warrant or Right, Expiration Period Following FDA Clearance					60 days
Common Stock Warrants [Member] | Placement Agency Sale - Prospectus Supplement of July 12, 2018 [Member]
Stock Issued During Period, Shares, New Issues			216,618
Class of Warrant or Right, Number of Securities Called by Warrants or Rights			108,309	108,309
Class of Warrant or Right, Exercise Price of Warrants or Rights			$ 15.80	$ 15.80
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right			1
Common Stock Warrants [Member] | November Public Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights		2,875,000
Class of Warrant or Right, Exercise Price of Warrants or Rights		$ 4
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right		1
Class of Warrant or Right, Outstanding							876,840			876,840
Class of Warrant or Right, Exercised										1,998,079
Proceeds from Warrant Exercises										$ 8,000,000
Preferred Stock Warrants [Member] | November Public Offering [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right		250
Class of Warrant or Right, Outstanding											2,875,000
Common Stock [Member]
Stock Issued During Period, Shares, New Issues		728,500
Stock Issued During Period, Shares, Employee Stock Purchase Plans										36,087	4,401
Common Stock [Member] | Placement Agency Sale - Prospectus Supplement of July 12, 2018 [Member]
Stock Issued During Period, Shares, New Issues				216,618
Common Stock [Member] | November Public Offering [Member]
Stock Issued During Period, Shares, New Issues		728,500
Conversion of Series B Preferred Stock into Common Stock [Member]
Conversion of Stock, Shares Converted										1,523
Conversion of Stock, Shares Issued										380,750
Conversion of Series C Preferred Stock into Common Stock [Member]
Conversion of Stock, Shares Converted										2,170
Conversion of Stock, Shares Issued										542,500
Series A Preferred Stock [Member]
Preferred Stock, Shares Outstanding, Ending Balance							48,325			48,325
Dividends, Preferred Stock, Total										$ 3,600,000	$ 2,900,000
Stock Issued During Period, Shares, New Issues					41,800
Series A Preferred Stock [Member] | CRG [Member]
Preferred Stock Dividends, Shares							3,580	2,945
Series B Preferred Stock [Member]
Preferred Stock, Par or Stated Value Per Share					$ 0.001
Preferred Stock, Shares Outstanding, Ending Balance							178			178
Stock Issued During Period, Shares, New Issues					17,979
Preferred Stock Deemed Dividend Arising from Beneficial Conversion Feature					$ 5,200,000
Series B Preferred Stock [Member] | Common Stock [Member]
Stock Issued During Period, Shares, New Issues
Series C Preferred Stock [Member]
Preferred Stock, Par or Stated Value Per Share		$ 0.001
Preferred Stock, Shares Outstanding, Ending Balance											2,170
Stock Issued During Period, Shares, New Issues		8,586
Proceeds from Issuance or Sale of Equity, Net		$ 10,200,000
Series C Preferred Stock [Member] | November Public Offering [Member]
Stock Issued During Period, Shares, New Issues		8,586
Proceeds from Issuance or Sale of Equity, Net		$ 10,200,000
Series C Preferred Stock [Member] | Common Stock [Member]
Stock Issued During Period, Shares, New Issues											728,500
Conversion of Principal Amount of Senior Secured Loan to Newly Authorized Series A Preferred Stock [Member] | Series A Preferred Stock [Member]
Convertible Preferred Stock, Shares of Common Stock Issuable on Conversion					2,090,000	2,090,000
Preferred Stock, Dividend Rate, Percentage						8.00%
CRG [Member] | Conversion of Principal Amount of Senior Secured Loan to Newly Authorized Series A Preferred Stock [Member]
Debt Conversion, Original Debt, Amount					$ 38,000,000	$ 38,000,000
Debt Conversion, Fees and Prepayment Premium Amount						3,800,000
Debt Conversion, Converted Instrument, Amount					$ 41,800,000	$ 41,800,000