A microfluidic device comprising: a fluid separation subcircuit comprising: a) a microfluidic channel having a first end and a second end; b) a sample inlet fluidly connected to the first end of the microfluidic channel configured for receiving a blood sample; c) a composite membrane interposed between the sample inlet and the first end of the microfluidic channel, wherein the composite membrane is capable of activating blood coagulation and removing selected particles from the blood; and d) an on-device pump fluidly connected to the second end of the microfluidic channel; and a solute mixing subcircuit comprising: i) a serpentine mixing channel, said mixing channel having a first end and a second end and having a critical length for enabling solute mixture by diffusion; ii) a first and second intake channel fluidly joined to said first end of said mixing channel at a staging union; said first intake channel for conveying a first fluid and said second channel for conveying a second fluid; wherein said staging union is configured with micro-passive valves for simultaneously releasing said first and second fluids into said mixing channel; iii) a downstream channel fluidly joined to the second end of said mixing channel, wherein the downstream channel has a width greater than the width of the mixing channel; iv) a pump for controlledly initiating fluid flow across said micro-passive valve, wherein said pump is fluidly connected to said downstream channel, and initiates flow by a suction stroke; and v) a vent terminating said downstream channel. a fluid separation subcircuit comprising: a) a microfluidic channel having a first end and a second end; b) a sample inlet fluidly connected to the first end of the microfluidic channel configured for receiving a blood sample; c) a composite membrane interposed between the sample inlet and the first end of the microfluidic channel, wherein the composite membrane is capable of activating blood coagulation and removing selected particles from the blood; and d) an on-device pump fluidly connected to the second end of the microfluidic channel; and a) a microfluidic channel having a first end and a second end; b) a sample inlet fluidly connected to the first end of the microfluidic channel configured for receiving a blood sample; c) a composite membrane interposed between the sample inlet and the first end of the microfluidic channel, wherein the composite membrane is capable of activating blood coagulation and removing selected particles from the blood; and d) an on-device pump fluidly connected to the second end of the microfluidic channel; and a solute mixing subcircuit comprising: i) a serpentine mixing channel, said mixing channel having a first end and a second end and having a critical length for enabling solute mixture by diffusion; ii) a first and second intake channel fluidly joined to said first end of said mixing channel at a staging union; said first intake channel for conveying a first fluid and said second channel for conveying a second fluid; wherein said staging union is configured with micro-passive valves for simultaneously releasing said first and second fluids into said mixing channel; iii) a downstream channel fluidly joined to the second end of said mixing channel, wherein the downstream channel has a width greater than the width of the mixing channel; iv) a pump for controlledly initiating fluid flow across said micro-passive valve, wherein said pump is fluidly connected to said downstream channel, and initiates flow by a suction stroke; and v) a vent terminating said downstream channel. i) a serpentine mixing channel, said mixing channel having a first end and a second end and having a critical length for enabling solute mixture by diffusion; ii) a first and second intake channel fluidly joined to said first end of said mixing channel at a staging union; said first intake channel for conveying a first fluid and said second channel for conveying a second fluid; wherein said staging union is configured with micro-passive valves for simultaneously releasing said first and second fluids into said mixing channel; iii) a downstream channel fluidly joined to the second end of said mixing channel, wherein the downstream channel has a width greater than the width of the mixing channel; iv) a pump for controlledly initiating fluid flow across said micro-passive valve, wherein said pump is fluidly connected to said downstream channel, and initiates flow by a suction stroke; and v) a vent terminating said downstream channel.