Note 8 - Common Stock - Summary of Outstanding Warrants (Details) - $ / shares	6 Months Ended
	Jun. 30, 2018	May 22, 2017	Jun. 20, 2016
Expiration Date
Exercise Price (in dollars per share)
Shares Outstanding Under Warrants (in shares)	642,622
September 2014 Issuance Through Private Offering [Member]
Expiration Date	Sep. 23, 2019
Exercise Price (in dollars per share)	$ 4.24
Shares Outstanding Under Warrants (in shares)	86,831
Vendors and Contractor Warrants, October 2014 Issuance [Member]
Expiration Date	Oct. 13, 2019
Exercise Price (in dollars per share)	$ 4.24
Shares Outstanding Under Warrants (in shares)	29,000
Vendors and Contractor Warrants, November 2014 Issuance [Member]
Expiration Date	Nov. 12, 2019
Exercise Price (in dollars per share)	$ 4.24
Shares Outstanding Under Warrants (in shares)	12,500
Employee Performance Bonus Warrants, February 2015 Issuance [Member]
Expiration Date	Feb. 17, 2025
Exercise Price (in dollars per share)	$ 4
Shares Outstanding Under Warrants (in shares)	75,697
Contractor Warrants, March 2015 Issuance [Member]
Expiration Date	Mar. 26, 2025
Exercise Price (in dollars per share)	$ 2.72
Shares Outstanding Under Warrants (in shares)	1,454
Contractor Warrants, May 2015 Issuance [Member]
Expiration Date	May 12, 2025
Exercise Price (in dollars per share)	$ 4.24
Shares Outstanding Under Warrants (in shares)	36,229
May 2015 Issuance Second Contractor [Member]
Expiration Date	May 17, 2020
Exercise Price (in dollars per share)	$ 4.24
Shares Outstanding Under Warrants (in shares)	21,585
Performance Bonus Warrants, December 2015 Issuance [Member]
Expiration Date	Dec. 16, 2025
Exercise Price (in dollars per share)	$ 5.60
Shares Outstanding Under Warrants (in shares)	26,875
Distributor Warrants, April 2016 Issuance [Member]
Expiration Date	Apr. 01, 2026
Exercise Price (in dollars per share)	$ 6.08
Shares Outstanding Under Warrants (in shares)	25,000
Contractor Warrants, May 2016 Issuance [Member]
Expiration Date	May 11, 2021
Exercise Price (in dollars per share)	$ 7.74
Shares Outstanding Under Warrants (in shares)	5,000
June 2016 Issuance Related to 2016 Loan Agreement [Member]
Expiration Date	Jun. 20, 2026
Exercise Price (in dollars per share)	$ 4.98		$ 4.98
Shares Outstanding Under Warrants (in shares)	100,402		100,402
May 2017 Issuance Related to 2017 Loan Agreement [Member]
Expiration Date	May 25, 2027
Exercise Price (in dollars per share)	$ 9.50	$ 9.50
Shares Outstanding Under Warrants (in shares)	222,049	222,049