1 research outputs found
Development of piezoresistive sensors for biomedical applications
Tese de doutoramento em Engenharia Electrónica Industrial e de ComputadoresIn the last decades there has been an increase in sensing systems applied in a
variety of situations with a large variety of sensor ranges. This represents a growing
area with high potential.
One of the areas of sensor development that require a great deal of attention is the
area of sensor for biomedical applications and biosensors.
These sensors have to overcome a number of challenges and limitations inherent to
the environment where they are introduced. These difficulties lead to the necessity
of using new materials and new techniques for their construction together with the
more traditional materials, e.g. silicon based, which have already proven their
potential in this area. Among the various materials, polymers have proven to be a
good choice, due to a set of advantages such as simple processing, flexibility and
facility of being obtained in different shapes. Therefore it is interesting to fabricate
polymer based piezoresistive sensors for functional coatings of implantable hip
prosthesis.
These sensors will allow coating the prosthesis and provide new functionalities to
the implants such as the possibility to measure forces and deformations between the
prosthesis and the bone and therefore improving the postoperative diagnostic.
In this works, a model of hip prosthesis with coated sensors was developed. For this
purpose, flexible piezoresistive sensors have been developed that allow being
implanted. Strain sensors were fabricated based on thin films of n+-nc-si.H by the
technique of hot-wire chemical vapor deposition at a temperature of 150 ºC on a
polymeric substrate, using the lithographic technique to construct the various layers
of the sensors. The sensor has a gauge factor of -28 for low frequency deformation
cycles. In the last decades there has been an increase in sensing systems applied in a
variety of situations with a large variety of sensor ranges. This represents a growing
area with high potential.
One of the areas of sensor development that require a great deal of attention is the
area of sensor for biomedical applications and biosensors.
These sensors have to overcome a number of challenges and limitations inherent to
the environment where they are introduced. These difficulties lead to the necessity
of using new materials and new techniques for their construction together with the
more traditional materials, e.g. silicon based, which have already proven their
potential in this area. Among the various materials, polymers have proven to be a
good choice, due to a set of advantages such as simple processing, flexibility and
facility of being obtained in different shapes. Therefore it is interesting to fabricate
polymer based piezoresistive sensors for functional coatings of implantable hip
prosthesis.
These sensors will allow coating the prosthesis and provide new functionalities to
the implants such as the possibility to measure forces and deformations between the
prosthesis and the bone and therefore improving the postoperative diagnostic.
In this works, a model of hip prosthesis with coated sensors was developed. For this
purpose, flexible piezoresistive sensors have been developed that allow being
implanted. Strain sensors were fabricated based on thin films of n+-nc-si.H by the
technique of hot-wire chemical vapor deposition at a temperature of 150 ºC on a
polymeric substrate, using the lithographic technique to construct the various layers
of the sensors. The sensor has a gauge factor of -28 for low frequency deformation
cycles.In the last decades there has been an increase in sensing systems applied in a
variety of situations with a large variety of sensor ranges. This represents a growing
area with high potential.
One of the areas of sensor development that require a great deal of attention is the
area of sensor for biomedical applications and biosensors.
These sensors have to overcome a number of challenges and limitations inherent to
the environment where they are introduced. These difficulties lead to the necessity
of using new materials and new techniques for their construction together with the
more traditional materials, e.g. silicon based, which have already proven their
potential in this area. Among the various materials, polymers have proven to be a
good choice, due to a set of advantages such as simple processing, flexibility and
facility of being obtained in different shapes. Therefore it is interesting to fabricate
polymer based piezoresistive sensors for functional coatings of implantable hip
prosthesis.
These sensors will allow coating the prosthesis and provide new functionalities to
the implants such as the possibility to measure forces and deformations between the
prosthesis and the bone and therefore improving the postoperative diagnostic.
In this works, a model of hip prosthesis with coated sensors was developed. For this
purpose, flexible piezoresistive sensors have been developed that allow being
implanted. Strain sensors were fabricated based on thin films of n+-nc-si.H by the
technique of hot-wire chemical vapor deposition at a temperature of 150 ºC on a
polymeric substrate, using the lithographic technique to construct the various layers
of the sensors. The sensor has a gauge factor of -28 for low frequency deformation
cycles. Sensors with larger flexibility were also developed though inkjet printing
technique. Various configurations and materials were used to evaluate which
materials are most appropriate for these types of sensors. Sensors with a gauge
factor of approximately 2.5 for an active layer of PeDOT were obtained. A sensor
matrix of 4 x 5 sensors was fabricated with an active area of 1.8 x 1.5 mm2 per
sensor. These sensors were subjected to a set of electromechanical tests to evaluate its
performance in situations close to end use. So the prosthesis was coated with the
various sensors, cemented and subjected to deformation cycles for three levels of
force according to standard ISO7206.
An adaptive system read-out electronic circuit was developed and built that allows
reading piezoresistive sensors with different characteristics. This system allows
measuring a matrix of 8x8 sensors, but can be scaled to a large number of sensors.
The readable range of the system is between 50 Ω and 100 kΩ according to the
needs of the sensors being implanted.
The total area of the circuit is 135 mm2, according to the requirements of a circuit
to be used in in-vivo applications. An energy management system was also
implemented that allows to activate and deactivate parts of the circuit when they are
not needed, reducing the energy consumption. The system was validated by
measuring a matrix of sensors with different characteristics.
Finally, simulations were performed in order to evaluate the best options for the
development of a wireless communications system. Three possible operation
frequency ranges were used for three types of standard antennas. The
communication system was introduced into a model simulating the characteristics
of the various layers that constitute the human body. These simulations allow evaluate the frequency range most appropriate for
implantable devices, the most appropriate antenna and the best location within the
body. So the frequency chosen for the implementation was 868 Mhz for a Inverted-
F antenna (IFA).
In conclusion, the key elements for the implementations of an instrumented hip
prosthesis were development and validated. The developed and/or simulated
elements, including sensors, circuits for reading and communication system can
also be used in other applications due to characteristics.These simulations allow evaluate the frequency range most appropriate for
implantable devices, the most appropriate antenna and the best location within the
body. So the frequency chosen for the implementation was 868 Mhz for a Inverted-
F antenna (IFA).
In conclusion, the key elements for the implementations of an instrumented hip
prosthesis were development and validated. The developed and/or simulated
elements, including sensors, circuits for reading and communication system can
also be used in other applications due to characteristics. Neste trabalho foi desenvolvido um modelo de prótese de anca com
implementação de sensores. Para atingir esse objectivo, foram desenvolvidos
sensores piezoresitivos flexíveis que permitam ser implantados. Assim foram
fabricados sensores de deformação baseados em filmes finos de n+-nc-si.H pela
técnica de hot-wire chemical vapor deposition a uma temperatura de 150ºC sobre
um substrato polimérico. Recorreu-se a técnica de litografia para construir as várias
camadas do sensor. Os sensores apresentam um gauge factor de -28, para ciclos de
baixa frequência em testes de four-point-bending.
Foram ainda desenvolvidos sensores com uma maior flexibilidade através da
técnica de inkjet printing. Para esse desenvolvimento foram usadas várias
configurações e materiais, para avaliar quais os materiais mais adequados para este
tipo de sensores. Na caracterização destes sensores obteve-se um gauge factor de
aproximadamente 2.5 para uma camada ativa de PeDOT. Com os melhores sensores obtidos foram construídas matrizes de 4 x 5 sensores que apresentam uma
área ativa de 1.8 x 1.5mm2 por sensor.
Estes sensores foram sujeitos a um conjunto de ensaios electromecânicos, para
avaliar o seu desempenho em situações próximas da utilização final. Desta forma
foi revestida uma prótese com os diferentes sensores, cimentada e sujeita a ciclos de
deformação para três níveis de força, segundo a norma ISO7206.
Foi desenvolvido e construído um sistema de leitura adaptável que permite medir
sensores piezoresistivos com diferentes características entre eles. Este sistema
permite medir uma matriz de 8x8 sensores, mas pode ser escalada para um número
maior de sensores. A gama de leitura do sistema varia entre 50 Ω e 100 kΩ, de
acordo com as necessidades dos sensores a serem implementados.
A área total deste circuito é de 135 mm2, de acordo com as necessidades de um
circuito a ser utilizado em aplicações in-vivo. Foi também implementado um
sistema de gestão de energia que permite ativar e desativar partes do circuito
quando estas não são necessárias, permitindo, desta forma, reduzir os consumos de
energia. O sistema foi validado através da medição de uma matriz de sensores com
diferentes características. foram realizadas simulações de forma a avaliar as melhores opções
para o desenvolvimento do sistema de comunicação sem fios. Foram usadas três
possíveis gamas de frequência de operação para três tipos de antenas standard. O
sistema de comunicação foi introduzido num modelo simulando as características
das várias camadas que constituem o corpo humano.
Estas simulações permitem aferir a gama de frequências mais adequadas para os
dispositivos implantáveis, a antena mais adequada e a sua melhor localização, pois
foi verificado como as várias camadas que constituem o corpo humano influenciam
a comunicação. Assim, a frequência escolhida para a implementação foi de 868
MHz e a antena foi a IFA.
Em conclusão, os elementos principais para a implementação de uma prótese de
anca instrumentada, foram desenvolvidos e validados. Os elementos desenvolvidos
e/ou simulados, incluindo os sensores, circuitos de leitura e sistema de
comunicação, poderão igualmente ser utilizados em outras aplicações devido às suas boas características