本研究ではピエゾ抵抗型カンチレバーを用いたMEMS差圧センサをアイマスクに組み込むことで，アイマスク型センサを製作し，非侵襲的でかつ簡易的なセットアップで睡眠時の脈波と呼吸波を同時に計測することを目的とする．そのために，提案するセンサ構造を試作し，実際の睡眠時の応答を評価した．センサの原理として，鼻背部分の直下にある血管の脈波によって鼻背表面が振動するため，これを圧力センサによって検出することで脈波を計測する．一方で，吸気・呼気運動によって、鼻腔に空気が流れるため，その流れによって同様に鼻背表面が振動する．流れによる振動の周波数帯は脈波による振動に対して十分に大きいため，周波数成分を分けることで呼気速度を算出できる．
設計・製作したデバイスは2つのチャンバ，シリコーンシート，カバー，カンチレバー素子の5つの部品からなる．高感度化のため，チャンバは無駄がないように容積を小さくし，シリコーンシートの厚さは0.1 mmとした．また，チャンバの変形防止および鼻の形状に合わせた形状を実現するためにチャンバとシリコーンカバーは光学式3Dプリンタを用いて製作した．製作したデバイスはアイマスクに縫い付けた．センサ応答は，Bluetooth通信によって無線で計測することができる．
評価実験として，アイマスク型センサを装着し，睡眠時の計測を行った．サンプリングレートは100 Hzとし，計測時間は1時間とした．計測したデータから脈波の周波数は0.9~1.5 Hz, 呼吸波の周波数は0.1~0.2 Hzと分かった．そのため，計測データに対して，4 Hzのローパスフィルタで脈波を，0.2 Hzのローパスフィルタで呼吸波を抽出した．抽出された脈波には脈波特有のP, Q, R, S, T波を確認できた．呼吸波が呼気振動の周期に一致していることが確認でき，この脈波および呼吸波の各ピーク間隔の平均をとることで脈波および呼吸波の周期はそれぞれ0.82 s, 5.55 sと求められ，つまり，脈拍は73 bpm, 呼気速度は 0.8 bpmと算出できた．
The purpose of this study is to fabricate an eye mask-type sensor by incorporating a MEMS differential pressure sensor with a piezoresistive cantilever into an eye mask to simultaneously measure pulse wave and respiratory wave during sleep in a non-invasive and simple setup. For this purpose, we fabricated a prototype of the proposed sensor structure and evaluated its response during actual sleep. The principle of the sensor is that the dorsal surface of the nose vibrates due to the pulse wave of blood vessels located just below the dorsal surface of the nose, which is detected by a pressure sensor to measure the pulse wave. On the other hand, air flows into the nasal cavity during inhalation and exhalation, and the flow causes the dorsal surface of the nose to vibrate in the same manner. The frequency band of the vibration caused by the flow is sufficiently large compared to the vibration caused by the pulse wave, so the expiratory velocity can be calculated by separating the frequency components.
The device designed and fabricated consists of five parts: two chambers, a silicone sheet, a cover, and a cantilever element. To achieve high sensitivity, the chambers were made small to avoid waste, and the thickness of the silicone sheet was 0.1 mm. The chamber and silicone cover were fabricated using an optical 3D printer in order to prevent deformation of the chamber and to realize a shape that matches the shape of the nose. The fabricated device was sewn onto an eye mask. The sensor response can be measured wirelessly via Bluetooth communication.
As an evaluation experiment, we measured the sensor response during sleep wearing the eye mask. The sampling rate was 100 Hz and the measurement time was one hour. From the measured data, the pulse wave frequency was found to be 0.9~1.5 Hz, and the respiratory wave frequency 0.1~0.2 Hz. Therefore, a 4 Hz low-pass filter was used to extract the pulse wave from the measured data, and a 0.2 Hz low-pass filter was used to extract the respiratory wave. P, Q, R, S, and T waves, which are characteristic of pulse waves, were identified in the extracted pulse waves. By taking the average of the peak intervals of the pulse wave and the respiratory wave, the period of the pulse wave and the respiratory wave were calculated to be 0.82 s and 5.55 s, respectively, which means that the pulse rate is 73 bpm and the expiratory velocity is 0.8 bpm.
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