Motivation: According to American Cancer Society, breast cancer continues to be the second leading cause of cancer-related female deaths in the US. The earlier the cancer is detected, the lower risk for the patients and higher is the probability of cure. The importance of early cancer detection in improved diagnosis and treatment has prompted considerable research interest in quantifying the state and progression of cancer. Mechanical phenotyping has been demonstrated to be an effective quantitative biomarker for characterizing the state of malignancy in cells and tissue. Advances in cancer biomechanics research has been supplemented by a surge in recent development of mechanical property measurement techniques at the micro and nano scale. The existing technique like capacitive force sensor or Atomic Force Microscopy (AFM), used for mechanical characterization of tissue suffers from limitations like complex microfabrication and read-out electronics, bulky optics, and inability to be used in opaque liquids. In addition, the AFM is also limited by its low throughput. Commercial AFMs typically use a single cantilever to probe discrete locations on the sample surface, which becomes cumbersome for large specimens such as histopathological tissue. Conventional AFM stages provide a limited range of travel (around 100 mm), which necessitates use of manual positioning systems to align the AFM probe and specimens. As such, piezoresistive sensing mechanisms offer an attractive alternative to the aforementioned techniques. Piezoresistive sensors can be used in opaque liquids and does not require complex readout electronics. In addition, multiple piezoresistive cantilevers can be microfabricated in an array-format, which considerably improves throughput and offers a cost effective approach to quantifying biomaterial mechanical properties.
The project comprises of fabricating and testing of MEMS-based piezoresistive microcantilevers with an SU-8 tip with the eventual goal of detecting benign and cancerous breast tissue in a high throughput manner.
Fabrication and Characterization of SOI and Polysilicon based Piezoresistive Microcantilever Force Sensor with SU-8 Tip and Development of Read-Out Electronics
The force sensor consists of a n-type cantilever on which p-type silicon piezoresistor is integrated. The cantilever is 260 mm long, 35 mm wide and 2 mm thick and the piezoresistors are 10 mm wide and 140 mm to 190 mm long. Each cantilever is having a built-in piezoresistor. The sensor is fabricated using standard silicon micro-machining technology and process flow is depicted in Fig. 1.
Fig. 1. Process flow for piezoresistive cantilever fabrication.
In case of sensor fabricated from polysilicon the microstructure of a thin film plays an important role in the evolution of residual stresses. To make a piezoresistor, boron was used as the spin-on-dopant. The drive-in temperature and time in our case was 1050 ⁰C and 1 h respectively. To study the material properties of the silicon film X-ray diffraction (XRD) measurements of the films were performed using Bruker D8 Advance powder diffractometer in glancing angle mode. Surface roughness of the as-deposited and annealed Si films was measured by AFM (Model: MFP-3D-BIOTM, Asylum Research) using contact mode. The residual stress of 2μm thick Si films was determined from the change in the radius of curvature of the wafer, before and after film deposition, using a temperature controlled film stress measurement system (Flexus F2320). Fig. 2 (left) shows the XRD patterns of as-deposited and annealed silicon film. The AFM plots are shown in Fig. 2 (right). Fig. 3 (left) shows the effect of annealing on residual stress of Si film. It was observed that the as-deposited silicon thin film has a compressive residual stress of about 340MPa. Annealing silicon films at 1050 ⁰C resulted in tensile stress of about 38.3MPa. In the sensor fabrication process the 200 nm thick Si3N4 plays an important role for compensating the stress in the device layer. Si3N4 also acts as a passivation layer even in a conductive solution like phosphate buffered saline (PBS), which is used for hydrating or preserving the tissues.
Fig. 2 (left) XRD of as-deposited and annealed Si films, (right) AFM image of (A) as-deposited, and (B) Si film annealed at 1050 ⁰C.
Fig. 3 (left) Effect of the sequential annealing process on residual stress for Si film, (right) Photograph of Piezoresistive Force sensor.
The photograph of the fabricated piezoresistive micro-cantilever sensor is shown in Fig. 3 (right). The scanning electron microscopy (SEM) image of the microcantilever with SU-8 tip is shown in Fig. 4 (left). The SEM of sensor fabricated without using 200 nm thick Si3N4 is shown in Fig. 4 (right). The spring constant of the fabricated cantilever was measured using AFM based reference cantilever technique. The sensor module with its electronics was developed for sensing the change in the resistance of the sensor with the indentation force. The signal conditioning circuit converts the change in resistance to voltage which is then applied to Data Acquisition Card (Sensoray Model 626) and is displayed on the computer screen. The detailed circuit diagram of the sensor module is shown in Fig. 5 (a) while, Fig. 5 (b) and (c) shows the AFM with sensor module and photograph of circuit respectively.
Fig. 4 (left) SEM image of sensor with SU-8 tip, (right) SEM image of bent cantilever.
Fig. 5. a) Circuit diagram of sensor module with sensor-head and electronic conditioning circuit, b) Photograph of AFM with Sensor Module, and c) photo of electronic conditioning circuit.
Sensitivity Measurements and Sensor Performance on Breast Tissue
We indented a cancerous breast tissue core with the fabricated cantilever. The optical photograph of microcantilever indenting the tissue is shown in Fig. 6 (left), while resistance change in the micro-cantilever as a function of its vertical displacement is shown in Fig. 6 (right). As observed in Fig. 6 (right), the sensor is responsive to tissue indentation in two separate spatial locations.
Fig. 6 (left) Optical image of piezoresistive cantilever indenting the breast tissue, (right) Sensor performance on breast tissue.
Our preliminary results demonstrated the feasibility of applying piezoresistive cantilevers as force sensors which can be used to characterize histological tissue samples. We believe that this novel approach will potentially provide a unique pathway to gaining insight into the biomechanical changes from the onset and progression of breast cancer and other diseases. In our future work, we plan to use the piezoresistive microcantilevers for characterizing the mechanical properties of benign and cancerous breast tissue samples. Also, an automated high-throughput electro-mechanical sensor for detection of cancerous breast tissues is envisaged.
Personnel: Hardik J. Pandya and Rajarshi Roy
Sponsor: NIH Grant R01CA161375.
- Hardik J. Pandya, Rajarshi Roy, Wenjin Chen, Marina A. Chekmareva, David J. Foran and Jaydev P. Desai, “Accurate characterization of benign and cancerous breast tissues: Aspecific patient studies using piezoresistive microcantilevers,” Biosensors and Bioelectronics, vol. 63, pp. 414–424, 2015; DOI: 10.1016/j.bios.2014.08.002.
- Hardik J. Pandya, Wenjin Chen, Lauri A. Goodell, David J. Foran and Jaydev P. Desai, ” Mechanical phenotyping of breast cancer using MEMS: a method to demarcate benign and cancerous breast tissues,” Lab on a Chip (Advanced Article), 2014; DOI: 10.1039/c4lc00594e.
- H. J. Pandya, Hyun Tae Kim, Rajarshi Roy and Jaydev P. Desai, “MEMS Based Low Cost Piezoresistive Microcantilever Force Sensor and Sensor Module,” Materials Science in Semiconductor Processing, vol. 19, pp. 163–173, 2014.
Refereed Conference Publications
- Hardik J. Pandya, Hyun Tae Kim, and Jaydev P. Desai, “A Microscale Piezoresistive Force Sensor for Nanoindentation of Biological Cells and Tissues” 2013 Dynamic Systems and Control Conference (DSCC 2013).