Just Purification
From Batch Buffer Exchange to Inline Conditioning: Direct Loading onto IEX Chromatography Using SSTFF
Steady-State TFF (SSTFF) turns buffer exchange from a separate batch operation into an inline process function, creating a new opportunity for connected downstream processing.
9/21/20264 min read
Process intensification is often approached by improving individual unit operations—higher chromatography productivity, increased membrane throughput, smaller equipment, or shorter cycle times. But another potentially more powerful approach is to reconsider why unit operations need to be separated in the first place.
Buffer exchange before ion exchange chromatography (IEX) is a good example.
In a conventional downstream process, feed may not have the appropriate pH or conductivity for direct loading onto an IEX column. TFF is therefore commonly used to perform buffer exchange as a separate batch operation. The conditioned product is collected in a holding vessel and subsequently transferred to chromatography.
The process typically looks like this:
Unconditioned Feed → Batch TFF/Buffer Exchange → Holding Tank → IEX Chromatography
This approach works, but it adds processing time, equipment, product hold, transfer operations, cleaning requirements, and footprint. More importantly, the intermediate tank creates a physical process boundary between TFF and chromatography.
What if that boundary could be removed?
Turning Buffer Exchange into an Inline Function
Lisure investigated a different process configuration using Steady-State Tangential Flow Filtration (SSTFF).
Instead of performing buffer exchange on an accumulated batch, SSTFF establishes a steady operating condition in which unconditioned feed enters continuously, buffer exchange occurs as part of the flowing process, and conditioned product exits continuously.
This makes possible a much simpler process configuration:
Unconditioned Feed → SSTFF Inline Buffer Exchange → Direct IEX Loading
The important difference is not simply that buffer exchange occurs faster. Buffer exchange has changed from an inventory-based batch operation into a flow-through conditioning function.
That distinction opens the possibility of directly connecting TFF with chromatography.
Traditional TFF and chromatography are not naturally easy to connect. TFF has its own recirculation, flow, and pressure requirements, while chromatography requires a controlled feed delivered within defined flow, pressure, pH, and conductivity conditions. The conventional holding tank decouples those two operations.
SSTFF addresses this problem by operating at constant flow and stable pressure while continuously generating an outlet with the required composition. If that outlet satisfies the inlet requirements of the chromatography operation, the intermediate tank may no longer be necessary.
Experimental Demonstration
The concept was experimentally demonstrated on Lisure's M++ UltraPilot platform.
The study used 0.5 mg/mL BSA at pH 7.5 in 20 mM phosphate buffer containing 0.2 M NaCl as the unconditioned feed. The exchange buffer was 20 mM phosphate buffer at pH 7.5.
SSTFF was performed using a 0.1 m² TFF cassette. The conditioned product stream was then loaded directly onto a 40-mm-diameter chromatography column packed with DEAE resin at a 5-cm bed height.
The SSTFF system operated with a feed flow of 25 mL/min and buffer flow of 100 mL/min, with inline pH and conductivity monitoring. Constant flow and stable pressure were maintained during operation.
This is an important aspect of the demonstration. The chromatography column did not receive the original feed composition. Instead, the original unconditioned feed entered the integrated process and was conditioned in real time by SSTFF immediately before chromatography.
There was no separate buffer-exchange batch and no intermediate product holding tank.
Comparable Chromatography Performance
Removing a conventional process step is valuable only if the performance of the downstream purification operation can be maintained.
In the study, no significant difference was observed in breakthrough behavior, elution peak shape, or recovery between the SSTFF-inline configuration and the control. The reported recovery in the elution fraction was 98.6%.
The results demonstrate the basic feasibility of the concept: SSTFF can perform buffer conditioning in real time while generating a product stream suitable for direct IEX loading.
At the process level, the configuration also shortened overall processing time and eliminated the intermediate holding tank, associated piping, product transfer, and tank/CIP-related cleaning requirements.
The Bigger Opportunity: Connecting Unit Operations
The larger significance of this work goes beyond buffer exchange.
In conventional bioprocessing, tanks are frequently used to connect operations that have different operating characteristics. They provide an effective engineering solution—but they also create material accumulation.
Material must be collected, held, transferred, and sometimes sampled or released before the next operation proceeds.
SSTFF suggests another approach.
Rather than asking:
“How can we make the buffer-exchange batch faster?”
we can ask:
“Can the upstream operation continuously generate exactly the conditions required by the downstream operation?”
If the answer is yes, an intermediate batch operation may become unnecessary.
This leads to a broader engineering principle for connected processing:
Two unit operations can potentially be directly connected when the upstream operation continuously provides the boundary conditions required by the downstream operation.
For SSTFF followed by IEX chromatography, those boundary conditions include composition, pH, conductivity, flow, pressure, and stability.
This is why SSTFF should not be viewed simply as another TFF configuration. In this application, SSTFF becomes an interface technology between unit operations.
From Material Accumulation to Material Flow
This way of thinking also changes how process intensification can be approached.
A downstream process can contain highly optimized individual operations and still require substantial time and infrastructure because product repeatedly accumulates between them.
Eliminating unnecessary intermediate inventories can therefore create benefits that cannot be achieved simply by making individual pieces of equipment faster.
The SSTFF experiment provides a practical example:
Conventional process
TFF → Accumulate → Hold → Transfer → Chromatography
SSTFF integrated process
SSTFF → Direct Flow → Chromatography
The difference is architectural.
And that architecture can potentially translate into shorter processing time, smaller equipment footprint, fewer vessels and transfers, reduced cleaning requirements, and simpler operations.
A Building Block for Connected Downstream Processing
This study demonstrates a specific application: inline buffer exchange enabling direct loading of initially unconditioned feed onto IEX chromatography.
But the underlying concept has broader implications.
Many interfaces in downstream processing exist because the output conditions of one operation do not naturally match the input requirements of the next. Conditioning functions—including buffer exchange, dilution, concentration, pH adjustment, and conductivity adjustment—are therefore important targets when designing more integrated processes.
SSTFF provides one example of how such a conditioning function can move from a separate batch operation into the material flow itself.
The experimental results demonstrated real-time buffer conditioning, direct connection to IEX chromatography, comparable chromatography performance, and elimination of the intermediate holding tank.
The larger opportunity is a different way of thinking about downstream process design:
From batch conditioning to inline conditioning.
From material accumulation to material flow.
From individually optimized unit operations to directly connected processing.
That may ultimately be where SSTFF makes its greatest contribution to process intensification and connected biomanufacturing.