Senior Secured Note Payable

The Company has a note payable to Brainlab AG (the “Brainlab Note”) in the principal amount of $4,289,444. Interest accrues at 5.5% per year. The Brainlab Note matures in April 2016, and principal and accrued interest is payable in a single installment upon maturity. The Brainlab Note is secured by a senior security interest in the assets of the Company. The original discount associated with the Brainlab Note represented the difference between the fair value and the principal amount of the note at the time the note was modified in March 2013. The unamortized discount at June 30, 2015 and December 31, 2014 was $171,848 and $271,305, respectively. The discount is being amortized to interest expense using the effective interest method over the term of the Brainlab Note.

2014 Junior Secured 12% Notes Payable

In March 2014, the Company entered into securities purchase agreements for the private placement of (i) 12% second-priority secured non-convertible promissory notes maturing in 2019 (the “2014 Secured Notes”) and (ii) warrants to purchase 0.3 shares of the Company’s common stock for each dollar in principal amount of the 2014 Secured Notes sold by the Company. Pursuant to those securities purchase agreements, the Company sold 2014 Secured Notes in a total aggregate principal amount of $3,725,000, together with warrants to purchase up to 1,117,500 shares of common stock, for aggregate gross proceeds of $3,725,000, before placement agent commissions and other expenses.

The 2014 Secured Notes have a five-year term, and they bear interest at a rate of 12% per year, payable semi-annually, in arrears, on each six-month and one-year anniversary of the issuance date. The 2014 Secured Notes are not convertible into shares of the Company’s common stock. Following the third anniversary of the issuance date, the 2014 Secured Notes may be prepaid, without penalty or premium, provided that all principal and unpaid accrued interest under all 2014 Secured Notes is prepaid at the same time. Prior to the third anniversary of the issuance date, the Company may prepay all, but not less than all, of the principal and unpaid accrued interest under the 2014 Secured Notes at any time, subject to the Company’s payment of the additional prepayment premium stated in the notes. The 2014 Secured Notes are secured by a security interest in the Company’s property and assets, which security interest is junior and subordinate to the security interest that secures Brainlab note.

The warrants issued to the investors are exercisable, in full or in part, at any time prior to the fifth anniversary of the issuance date, at an exercise price of $1.75 per share, subject to adjustment from time to time for stock splits or combinations, stock dividends, stock distributions, recapitalizations and other similar transactions. Assumptions used in calculating the fair value of the warrants using the Black-Scholes valuation model were:

Under GAAP, the Company allocated the $3,725,000 in proceeds proportionately between the 2014 Secured Notes and the warrants issued to investors based on their relative fair values, with $413,057 being allocated to the fair value of the warrants, recorded as equity. The 2014 Secured Notes were recorded at the principal amount, less a discount equal to $413,057. This discount is being amortized to interest expense over the five year term of the 2014 Secured Notes using the effective interest method.

Non-employee directors of the Company purchased a total of $1,100,000 of the 2014 Secured Notes, either directly or through a trust. The Company’s placement agents earned cash commissions of $145,500 as well as warrants to purchase 72,750 shares of the Company’s common stock. The placement agent warrants have the same terms and conditions as the investor warrants. The placement agent cash commissions, the $30,210 fair value of the placement agent warrants, and other offering expenses totaling $76,186 were recorded as deferred financing costs and are classified as other assets. These deferred financing costs are being amortized to interest expense over the term of the 2014 Secured Notes using the effective interest method.

2010 Junior Secured Notes Payable

In November 2010, the Company issued units consisting of a junior secured note and one share of the Company’s common stock. An aggregate of 10,714,286 units were issued, and the Company received proceeds of $3,000,000 representing the aggregate principal amount of the junior secured notes. The junior secured notes mature in November 2020 and accrue interest at the rate of 3.5% per year. The junior secured notes are secured by a security interest in the assets of the Company, which security interest is junior and subordinate to the security interests that secure the Brainlab Note and the 2014 Secured Notes. All outstanding principal and interest on the junior secured notes will be due and payable in a single payment upon maturity.

Under GAAP, the Company allocated the $3,000,000 in proceeds from the sale of the units between the junior secured notes and the shares of common stock based on their relative fair values, with $2,775,300 being allocated to the value of the shares of common stock, which was recorded as equity. The junior secured notes were recorded at the principal amount of $3,000,000, less a discount equal to $2,775,300. This discount is being amortized to interest expense over the 10-year term of the notes using the effective interest method. The fair value of the notes was estimated based on an assumed market interest rate for notes of similar terms and risk. The fair value of the Company’s common stock was estimated by management using a market approach, with input from a third-party valuation specialist.

Four officers of the Company purchased an aggregate of 882,726 units in the offering for $247,164. In addition, three non-employee directors of the Company also purchased an aggregate of 567,203 units in the offering for $158,816.

Scheduled Notes Payable Maturities

Scheduled principal payments with respect to notes payable are summarized as follows: