Poly(lactic acid) (PLA) is a biodegradable polymer material used for the fabrication of objects by fused filament fabrication (FFF) 3D printing. FFF 3D printing technology has been quickly spreading over the past few years. An FFF-3D-printed object is formed from melted polymer extruded from a nozzle layer-by-layer. The mechanical properties of the object, and the changes in those properties as the object degrades, differ from the properties and changes observed in bulk objects. In this study we evaluated FFF-3D-printed objects by uniaxial tensile tests and four-point flexural tests to characterize the changes of three mechanical properties, namely, the maximum stress, elastic modulus, and breaking energy. Eight types of test pieces printed directly by an FFF 3D printer using two scan patterns and two interior fill percentages (IFPs) were tested by the aforesaid methods. The test pieces were immersed in saline and kept in an incubator at 37 °C for 30, 60, or 90 days before the mechanical testing. The changes in the mechanical properties differed largely between the test piece types. In some of the test pieces, transient increases in strength were observed before the immersion degraded the strength. Several of the test piece types were found to have superior specific strength in the tests. The results obtained in this research will be helpful for the design of PLA structures fabricated by FFF 3D printing.

The pH values of aqueous solutions are conventionally measured with pH-sensitive electrodes such as glass electrodes or ion-sensitive field-effect transistors (ISFETs) used in conjunction with Ag/AgCl reference electrodes and KCl solutions. The speed of pH measurement with these systems can be deficient, however, as the glass electrode responds slowly during measurements of sample solutions with low buffering capacities. Our group has constructed a new pH measurement system using a microfluidic device and ISFET sensors. The device has a channel with two inlets and one outlet, with a junction connected to a Y-shaped channel on the same plane. Two ISFET sensors and an Ag/AgCl pseudo reference electrode are fitted into the channel to construct a differential measurement device. A sample solution and baseline solution supplied into the inlets by gravity-driven pumps form a flowing liquid junction during measurement. The small size and fast response of the ISFET sensors enable measurement of about 2.0 mL of sample solution over a measurement period of 120 s. The 90% response time is within 2 s. The calibrated sensor signal exhibits a wide range (pH 1.68–10.0) of linearity with a correlation factor of 0.9997. The measurement error for all solutions tested, including diluted solutions, was 0.0343 ± 0.0974 pH (average error ± standard deviation (S.D.), n = 42). The new device developed in this research will serve as an innovative technology in the field of potentiometry.

A fused filament fabrication (FFF) 3D printer is a simple device capable of manufacturing three-dimensional structures in a series of easy steps. Commercial-level FFF 3D printers have spread rapidly in many fields in recent years. Poly(lactic acid) (PLA) is a biodegradable thermoplastic polymer used as a typical printing medium for FFF 3D printers. The FFF printer constructs an object with melted polymer extruded from a tiny scanning nozzle. The mechanical properties of FFF 3D structures printed with different scan patterns can therefore vary in accordance with the directions from which forces act upon them. The nozzle scan pattern also influences the deterioration of the mechanical properties of the structures in accordance with the degradation caused by the hydrolysis of PLA. In this study we conducted tensile tests to evaluate the strength characteristics of 3D printed test pieces formed from PLA using four different scan patterns: parallel, vertical, parallel-and-vertical, and cross-hatched at opposing diagonal angles to the tensile direction. We also formed test pieces by an injection molding method using the same material, for further comparison. We evaluated the deterioration of the test pieces after immersing them in saline for certain periods. After the test pieces formed by different nozzle scan patterns were immersed, they exhibited differences in the rates by which their maximum tensile stresses deteriorated and their masses increased through water uptake. The influences of the scan patterns could be classified into two types: the unidirectional scan pattern influence and bidirectional scan pattern influence. The data obtained in this research will be applied to structural design when the FFF 3D printer is employed for the fabrication of structures with PLA filament.

The liquid junction potential (LJP), the phenomenon that occurs when two electrolyte solutions of different composition come into contact, prevents accurate measurements in potentiometry. The effect of the LJP is usually remarkable in measurements of diluted solutions with low buffering capacities or low ion concentrations. Our group has constructed a simple method to eliminate the LJP by exerting spatiotemporal control of a liquid junction (LJ) formed between two solutions, a sample solution and a baseline solution (BLS), in a flow-through-type differential pH sensor probe. The method was contrived based on microfluidics. The sensor probe is a differential measurement system composed of two ion-sensitive field-effect transistors (ISFETs) and one Ag/AgCl electrode. With our new method, the border region of the sample solution and BLS is vibrated in order to mix solutions and suppress the overshoot after the sample solution is suctioned into the sensor probe. Compared to the conventional method without vibration, our method shortened the settling time from over two min to 15 s and reduced the measurement error by 86% to within 0.060 pH. This new method will be useful for improving the response characteristics and decreasing the measurement error of many apparatuses that use LJs.

Poly (lactic acid) (PLA), a biodegradable polymer, is widely used in medical applications, particularly for 3D-printed tissue engineering scaffolds. The fused filament fabrication (FFF) 3D printer is an available processing tool for PLA. The nozzle scan pattern and interior fill percentage (IFP) considerably influence the mechanical properties of formed structures and may have dominant effects on the rates at which the mechanical properties of PLA deteriorate. When the IFP is set to a low value, such as 80%, internal gaps form within the structure, leading to different deterioration patterns compared to structures formed under the IFP 100% condition. In this study, we fabricated test pieces with an FFF 3D printer using three different nozzle scan patterns. After immersing the test pieces in phosphate buffer saline (PBS) for up to 120 days, the water content was measured and the test pieces underwent tensile testing to determine the tensile strength, elastic modulus, and breaking energy. Both the deterioration rate and water uptake rate varied among the different nozzle scan patterns used for the fabrication. For the test pieces formed with internal gaps, the water uptake and deterioration proceeded in two stages. The deterioration rate of the structures with internal gaps was faster than that of the fully filled structures. The data obtained in this study will be useful for the design of PLA structures applied in tissue engineering.

PH is an essential parameter which express acidity or alkalinity, and the values are used in many fields such as basic sciences, medicine, and chemical industries. The pH values have been measured by the glass electrode method, however the operator suffers from the slow response and instability of the electrode. In contrast, ion-sensitive field-effect transistor (ISFET) is a semiconductor-based sensor device, and it has advantages such as small size and quick response to the solutions with low buffering capacities. For the handling of the solution, we employed the microfluidic technology, and ISFET sensor was inserted into its small channel. We have obtained quick response and successfully measured pH values. The present device showed high-linearity with the correlation factor of 0.9992 from the calibration curve. The measurement error of the 10 kinds of sample solutions with high and low-buffering capacities were -0.0690 ± 0.1556, and the response time was within 2 sec.