受洋紫荊果莢螺旋型態啟發之4D列印形狀變換

Abstract

洋紫荊(Bauhinia blackeana)常作為行道樹與園藝景觀來進行栽培，其果實以果莢的型式存在，並以如爆炸般的方式進行開裂將能量傳予種子以進行散播。根據野外觀察，洋紫荊果莢與其他豆科植物不同之處，在於開裂後果莢以「內捲」之型態存在，經過一段時間後則會翻轉成為「外翻」之姿態，具有兩種截然不同的型態，而目前並無研究針對此現象背後的力學機制進行探討。為了解洋紫荊果莢形成兩種獨特的螺旋型態的機制，本研究分別針對其分層結構、機械性質以及收縮方向進行觀察與量測，並將上述的實驗結果透過製作合成模型(Synthetic model)驗證果莢具有兩種螺旋型態的機制。 過去針對植物型態變化的研究多以水凝膠或是矽膠以灌模的方式製作合成模型，而灌模的過程繁雜，需另外製作模具使製作的時間成本高。而本研究藉由4D列印的方式製作合成模型，4D列印為3D列印技術的延伸，於3D列印中添加智能材料使模型受外界刺激後能再次變形，因此能藉由製造簡單的結構變形為複雜的結構，能夠節省許多的時間。此外由於受環境刺激後開始變形的概念與果莢型態變化的原理類似，所以適合以4D列印的方式進行型態模擬。本研究使用熔融沉積式(Fused deposition modeling, FDM)的3D列印機搭配PLA與形狀記憶線材SMP55進行4D列印模型的製作，並針對不同列印參數進行實驗與觀察該列印參數對模型變形的影響，最後以果莢的各項分析結果與列印參數對應以做出合成模型來驗證洋紫荊果莢的變形機制。 由實驗結果得知，洋紫荊果莢由內到外分別由纖維細胞、厚壁細胞、薄壁細胞以及外果皮厚壁細胞四種細胞組成，彼此之間的排列方向與收縮方向相互垂直，同時透過4D列印模型的類比成功模擬出洋紫荊果莢的兩種型態，確立了洋紫荊果莢的螺旋型態與其對應的形成機制：內捲為完整的四層結構，並且其螺旋方向由纖維細胞之收縮主導；由於薄壁細胞與厚壁細胞的分離使厚壁細胞得以收縮，而厚壁細胞的主要收縮方向與纖維細胞垂直，因此果莢形成螺旋方向相反的外翻型態，此時便轉換成由厚壁細胞主導。此外，本研究針對洋紫荊果莢於開裂時的能量進行探討，能量轉換效率範圍為4.81%至58.42%。轉換效率範圍雖廣，但與其他文獻之果實相比仍為較有效率之種子傳播方式，同時也透過4D列印的方式模擬出類似於果莢爆炸開裂的模式，於近乎瞬間即完成開裂過程。

Bauhinia blackeana is often cultivated as a street tree and for horticultural landscaping, and its fruit is in the form of seed pods that burst explosively, also called pod-shatter, to transfer energy to the seeds for dispersal. According to field observations, the difference between the seed pods of B. blackeana and other leguminous plants is that the seed pods maintain the form of 'inversion' after pod-shatter, and after a while, they transform into the form of 'eversion,' which means that the seed pods have two distinct helical shapes. However, no study in the literature has explored this phenomenon so far. To understand the underlying mechanics of the formation of two unique helical shapes of B. blackeana seed pods, we conducted a combined experimental and numerical study. We first measured the layered structure, mechanical properties, and shrinkage ratio of each layer of B. blackeana seed pods. We then created 4D-printed synthetic models that mimic the actual seed pod’s mechanical structure and properties, reproducing the two distinct helical shapes and providing insights into the formation mechanism. Past studies on plant shape transformation used hydrogel or silicone to make synthetic models by filling a mold. However, the filling process requires additional molds and is complicated and time-consuming. In this study, synthetic models were made by 4D printing. 4D printing is an extension of 3D printing technology. Smart materials are added to 3D printing so the model can be deformed again after being stimulated by heat. Therefore, 4D printing can save significant time by manufacturing simple structures and deforming them into complex ones. In addition, the concept of deformation after being stimulated by the environment is similar to the principle of seed pods shape transformation; it is suitable for shape simulation by 4D printing. A fused deposition modeling (FDM) 3D printer was used with PLA and a shape memory filament SMP55 to make a 4D printing model, and experiments were carried out for different printing parameters to observe the effect of the printing parameters on the deformation of the model. Finally, the analysis results of the seed pods correspond to the printing parameters to make a synthetic model to verify the deformation mechanism of the seed pods of B. blackeana. Our experimental results show that the seed pods of B. blackeana are composed of four layers of cells: fiber cells, sclerenchyma cells, parenchyma cells, and exocarp sclerenchyma cells. The arrangement and shrinkage directions of the tissues are perpendicular to each other. Furthermore, the analogy through 4D printing successfully simulates the two helical shapes of B. blackeana seed pods and confirms the corresponding mechanism of these two helical shapes: the 'inversion' is a complete four-layer structure, and its helical direction is dominated by the shrinkage of fiber cells. Because the separation of parenchyma cells and sclerenchyma cells allows sclerenchyma cells to shrink, and the main shrinkage direction of sclerenchyma cells is perpendicular to the fiber cells, the seed pods form 'eversion' with opposite helical directions. At this time, the 'eversion' is dominated by sclerenchyma cells. In addition, this study investigated the energy conversion of B. blackeana seed pods pod-shatter, and the energy conversion efficiency ranged from 4.81% to 58.42%. Although the conversion efficiency varies widely, it is still a more efficient way of seed dispersal than other plant species that utilize a similar mechanism. Moreover, we also demonstrate that the pod-shatter process of the 4D-printed models can occur and be complete within a short time, similar to actual seed pods in the field.