Note 10 - Stockholders' Equity (Deficit) (Details Textual) - USD ($) | Nov. 01, 2018 | Jul. 13, 2018 | Jul. 12, 2018 | Feb. 16, 2018 | Feb. 14, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Aug. 22, 2018 | Jan. 31, 2015 |
Preferred Stock, Shares Authorized | | | | | | 5,000,000 | 5,000,000 | | |
Preferred Stock, Par or Stated Value Per Share | | | | | | $ 0.001 | $ 0.001 | | |
Preferred Stock, Shares Issued, Total | | | | | | 45,671 | 0 | | |
Debt Conversion, Original Debt, Amount | | | | | | $ 42,794,000 | | | |
Preferred Stock, Shares Outstanding, Ending Balance | | | | | | 45,671 | 0 | | |
Proceeds from Issuance or Sale of Equity, Net | $ 10,200,000 | | $ 3,000,000 | $ 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 | | |
Common Stock, Par or Stated Value Per Share | | | | | | $ 0.001 | $ 0.001 | | |
Common Stock, Shares, Issued, Total | | | | | | 34,921,999 | 833,597 | | |
Class of Warrant or Right, Outstanding | | | | | | 47,575,393 | 53,803 | | |
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 | | | |
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense | | | | | | $ 0 | $ 0 | | |
Share Price | | | | | | $ 0.30 | | | |
Common Stock, Shares, Outstanding, Ending Balance | | | | | | 34,921,999 | 833,597 | | |
Employee Stock Option [Member] | | | | | | | | | |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total | | | | | | $ 430,000 | | | |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition | | | | | | 1 year 109 days | | | |
Restricted Stock Units (RSUs) [Member] | | | | | | | | | |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total | | | | | | $ 4,300,000 | | | |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition | | | | | | 2 years 182 days | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number, Ending Balance | | | | | | 2,940,662 | 5,089 | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Fair Value | | | | | | $ 900,000 | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value | | | | | | $ 1,500 | $ 97,000 | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | | | | | | $ 1.50 | $ 114.41 | | |
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 | | | | | | | | | 33,000 |
Share-based Compensation Arrangement by Share-based Payment Award, Additional Shares Reserved for Issuance, Annual | | | | | | | | | 42,250 |
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, Other Share Increase (Decrease) | | | | | | 41,674 | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized | | | | | | 3,000,000 | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | | | | | | 126,686 | | | |
Officer and Director Share Purchase Plan [Member] | | | | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized | | | | | | | | 200,000 | |
Stock Issued During Period, Shares, Employee Stock Purchase Plans | | | | | | 44,012 | | | |
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% | | | |
Class of Warrant or Right, Outstanding | | | | | | 53,803 | | | |
Class of Warrant or Right, Exercise Price of Warrants or Rights | | | | | | $ 504 | | | |
Common Stock Warrants, Issued with Series B Preferred Stock [Member] | | | | | | | | | |
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | | | | 17,979,000 | | 17,688,500 | | | |
Stock Issued During Period, Shares, Warrants Exercised | | | | | | 290,500 | | | |
Common Stock Warrants, Issued with Series B Preferred Stock, Warrant One [Member] | | | | | | | | | |
Class of Warrant or Right, Exercise Price of Warrants or Rights | | | | $ 0.40 | | | | | |
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 | | | | 500 | | | | | |
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 | | | | 500 | | | | | |
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] | | | | | | | | | |
Class of Warrant or Right, Exercise Price of Warrants or Rights | | $ 1.58 | $ 1.58 | | | | | | |
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | | 1,083,091 | 1,083,091 | | | 1,083,091 | | | |
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, Outstanding | | | | | | 28,750,000 | | | |
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ 0.40 | | | | | | | | |
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | 28,750,000 | | | | | | | | |
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right | 1 | | | | | | | | |
Preferred Stock Warrants [Member] | November Public Offering [Member] | | | | | | | | | |
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right | 2,500 | | | | | | | | |
Common Stock [Member] | | | | | | | | | |
Stock Issued During Period, Shares, New Issues | 7,285,000 | | | | | | 234,518 | | |
Stock Issued During Period, Shares, Warrants Exercised | | | | | | 290,500 | | | |
Stock Issued During Period, Shares, Employee Stock Purchase Plans | | | | | | 44,012 | | | |
Common Stock [Member] | Placement Agency Sale - Prospectus Supplement of July 12, 2018 [Member] | | | | | | | | | |
Stock Issued During Period, Shares, New Issues | | 2,166,180 | 2,166,180 | | | 2,166,180 | | | |
Common Stock [Member] | November Public Offering [Member] | | | | | | | | | |
Stock Issued During Period, Shares, New Issues | 7,285,000 | | | | | 7,285,000 | | | |
Conversion of Series B Preferred Stock into Common Stock [Member] | Common Stock [Member] | | | | | | | | | |
Conversion of Stock, Shares Issued | | | | | | 8,139,000 | | | |
Conversion of Series C Preferred Stock into Common Stock [Member] | Common Stock [Member] | | | | | | | | | |
Conversion of Stock, Shares Issued | | | | | | 16,040,000 | | | |
Series A Preferred Stock [Member] | | | | | | | | | |
Preferred Stock, Shares Outstanding, Ending Balance | | | | | | 41,800 | | | |
Dividends Payable | | | | | | $ 2,900,000 | | | |
Stock Issued During Period, Shares, New Issues | | | | 41,800 | | | | | |
Series B Preferred Stock [Member] | | | | | | | | | |
Preferred Stock, Par or Stated Value Per Share | | | | $ 0.001 | | | | | |
Preferred Stock, Shares Outstanding, Ending Balance | | | | | | 1,701 | | | |
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] | Conversion of Series B Preferred Stock into Common Stock [Member] | | | | | | | | | |
Conversion of Stock, Shares Converted | | | | | | 16,278 | | | |
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 | | | | | | | | |
Series C Preferred Stock [Member] | November Public Offering [Member] | | | | | | | | | |
Stock Issued During Period, Shares, New Issues | 8,586 | | | | | | | | |
Series C Preferred Stock [Member] | Conversion of Series C Preferred Stock into Common Stock [Member] | | | | | | | | | |
Conversion of Stock, Shares Converted | | | | | | 6,416 | | | |
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 | | | | 20,900,000 | 20,900,000 | | | | |
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 | $ 42,800,000 | | | | |