Quantitative force feedback for orthopaedic surgeons is crucial for the accurate positioning of implants during total hip and knee arthroplasty (THA and TKA respectively). During the trial stage of THA, the surgeon manually determines the implant size, positioning, and range of joint motion of the femoral (ball and stem) and acetabular (socket) parts of the hip implant using a trial acetabular liner, before discarding the liner and replacing it with the final liner. Poor implant positioning during this stage can lead to implant failure, which harms patient well-being and increases pressure on healthcare systems. As a result, incorporating force sensors within this trial hip liner during the trial stage of THA could improve surgical outcomes, by giving surgeons real-time quantitative force feedback to aid implant positioning, and by acting as a guidance tool for trainee surgeons. However, existing force sensors cannot be easily incorporated into the trial hip implant, due to the small and complex geometry of the joint, so there is a crucial need for an alternative technology to be developed.

Here, a novel thin, flexible, conformable, and robust microfluidic force sensor is developed which can be incorporated into the hip implant liner during the trial phase of total hip arthroplasty. The sensor comprises a thin, flexible polyimide (Kapton, PI) substrate, onto which silver electrodes are deposited using aerosol-jet printing (AJP). The substrateis bonded to a microfluidic chip, which comprises a flexible deformable elastomer with an embedded fluid reservoir and channel. When an external force is applied to the reservoir portion of the microfluidic chip, the reservoir is compressed and displaces fluid through the microfluidic channel. As the fluid is displaced through the channel, it overlaps with the electrodes, changing the dielectric properties of the channel, and therefore changing the electrical impedance measured by the electrodes.

The AJP printing parameters were optimised for the silver electrodes, and the resistance of the printed silver changed by up to 25% after 500 cycles of flexion. For the microfluidic chip, a study was done to compare the suitability of several elastomers such as polydimethylsiloxane (PDMS), Flexdym, and stereolithography 3D printing (SLA) 3D printing resins using a range of characterisation techniques. By conducting a series of T-peel adhesion tests, Flexdym was found to have the largest and most tuneable bonding strength to Kapton, compared to PDMS and SLA resin. Hyperelastic models were used to describe the mechanical behaviour based on tensile and compressive testing, with the Mooney-Rivlin 5-parameter model being the most suitable for all elastomers. Profilometry and contact angle goniometry indicated a high dependence of elastomer fabrication parameters on the wetting interaction between the liquid and the channel. Rheometry results indicate that PDMS has the shortest stress relaxation time, but its low stiffness makes it unsuitable for large applied forces, while the 3D printing resin has the opposite issue.

The sensor was calibrated using a linear motor, which consists of a stationary part with attached load cell and a translating part with an attached pressing arm. The sensor was mounted onto the stationary part, and a sinusoidal force with known frequency and amplitude was applied to the fluid reservoir using the pressing arm. The resulting impedance change was measured using an impedance analyser, and was determined to have a high dependence on the type of elastomer, the device dimensions, and the magnitude and frequency of the applied force. The required properties of the sensor, such as the maximum detectable force, were optimised by material choice and device design, both experimentally and using finite element modelling, and it was found that the sensors could be reliably calibrated for forces up to approximately 20N, which is a couple of orders of magnitude below the forces applied during THA and TKA.

To mimic the hip joint geometry, a novel trial hip implant liner which incorporates six microfluidic force sensors was designed using computer-aided design software, and prototypes were fabricated using SLA 3D printing from Flexible Resin and Durable Resin (Formlabs, United States). To calibrate the sensors within this prototype trial liner, a bespoke mechanical testing rig was developed using a combination of machined and SLA-printed components. The rig consisted of a polycarbonate base and an SLA-printed (Clear Resin, Formlabs) insert which housed the prototype hip liner. An aluminium rod containing a ceramic (aluminium oxide) femoral head was attached to a mechanical testing machine and used to apply forces of up to 1 kN to the prototype liner. The insert was rotated inside the rig to orient the liner from −30◦ to 30◦ to the femoral component of the test rig, to represent a surgeon applying forces at a range of angles to the liner during the trial phase of THA. By taking advantage of the force shielding effect of incorporating sensors into the stiff trial liner, the sensors were reliably calibrated up to approximately 1 kN. In order to assess balance in the test rig, the changes in capacitance were compared to finite element simulations of a symmetrical, balanced, ball and socket joint. Such a high force range is novel for a microfluidics-based force sensor, and so this technology is a potentially powerful surgical tool to guide orthopaedic surgeons.