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                                                                   EXHIBIT 10.18

                       FOURTH AMENDMENT TO LEASE AGREEMENT

THIS FOURTH AMENDMENT TO LEASE AGREEMENT (the "Fourth Amendment") is made and
entered into this 1st day of June, 2000, by and between FIRST INDUSTRIAL, L.P.,
a Delaware Limited Partnership ("Landlord"), and OPTICAL SENSORS INCORPORATED, a
Delaware corporation ("Tenant").

                                   WITNESSETH:

WHEREAS, Landlord and Tenant have heretofore entered into a certain Standard
Commercial Lease dated October 7, 1991 (the "Original Lease") pursuant to which
Landlord leased to Tenant and Tenant leased from Landlord certain premises
commonly known as 7615 Golden Triangle Drive, Suite A, Eden Prairie, Minnesota
(the "Premises"); and

WHEREAS, Landlord and Tenant entered into a certain First Amendment to Lease
Agreement dated April 26, 1996 (the "First Amendment"; the original Lease and
the First Amendment are hereinafter collectively referred to as the "Lease");
and

WHEREAS, Landlord and Tenant entered into a certain Second Amendment to Lease
Agreement dated April 14, 1997 (the "Second Amendment"; the Original Lease and
Second Amendment are hereinafter collectively referred to as the "Lease"); and

WHEREAS, Landlord and Tenant entered into a certain Third Amendment to Lease
Agreement dated September 3, 1999 (the "Third Amendment"; the Original Lease and
Second Amendment and Third Amendment are hereinafter collectively referred to as
the "Lease");

NOW, THEREFORE, for good and valuable consideration, the receipt and sufficiency
of which are hereby acknowledged, the parties hereto hereby agree as follows:

1.   Recitals. The foregoing recitals are hereby incorporated as if fully
     rewritten and restated in the body of this Fourth Amendment. All initially
     capitalized terms used but not defined herein shall have the meanings
     ascribed to such terms in the Lease.

2.   Term. The term of the Lease shall commence on June 1, 2000 and continue on
     a month to month basis with thirty (30) day written cancellation by either
     party. This Fourth Amendment to Lease shall expire May 31, 2001.

3.   Premises. The premises shall total 18,339 square feet; (7,906 square feet
     office, 8,205 square feet lab, 2,228 square feet warehouse) located at 7615
     Golden Triangle Drive, Suite A, as attached on Exhibit A.

4.   Tenant's Proportionate Share. Tenant's Proportionate Share of Operating
     Expenses shall be 14.56%.

5.   Base Rent. Effective June 1, 2000, the monthly Base Rent for the Premises
     shall be $13,101.76.
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6.   Full Force and Effect. Except as otherwise expressly set forth in this
     Fourth Amendment, all terms, provisions and covenants set forth in the
     Lease shall remain in full force and effect and are hereby ratified and
     confirmed as of the date hereof.

7.   Conflicts. In the event that any of the terms, covenants and conditions of
     this Fourth Amendment conflict with any of the terms, covenants and
     conditions of the Lease, the terms, covenants and conditions of this Fourth
     Amendment shall control.

IN WITNESS WHEREOF, the parties hereto have executed this Fourth Amendment as of
the date and year first above written.

LANDLORD: FIRST INDUSTRIAL, L.P.,      TENANT: OPTICAL SENSORS INCORPORATED,
a  Delaware  Limited Partnership       a Delaware corporation

By: First Industrial Realty Trust,
    Inc., a Maryland corporation,
    its general partner

By:                                    By:
   -------------------------------         -------------------------------------
Its:  Senior Regional Director         Its:  President and CEO



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